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The cadherin-catenin complex is essential for tissue morphogenesis during animal development. In cultured mammalian cells, p120 catenin (p120ctn) is an important regulator of cadherin-catenin complex function. However, information on the role of p120ctn family members in cadherin-dependent events in vivo is limited. We have examined the role of the single Caenorhabditis elegans p120ctn homologue JAC-1 (juxtamembrane domain [JMD]-associated catenin) during epidermal morphogenesis. Similar to other p120ctn family members, JAC-1 binds the JMD of the classical cadherin HMR-1, and GFP-tagged JAC-1 localizes to adherens junctions in an HMR-1-dependent manner. Surprisingly, depleting JAC-1 expression using RNA interference (RNAi) does not result in any obvious defects in embryonic or postembryonic development. However, jac-1(RNAi) does increase the severity and penetrance of morphogenetic defects caused by a hypomorphic mutation in the hmp-1/alpha-catenin gene. In these hmp-1 mutants, jac-1 depletion causes failure of the embryo to elongate into a worm-like shape, a process that involves contraction of the epidermis. Associated with failed elongation is the detachment of actin bundles from epidermal adherens junctions and failure to maintain cadherin in adherens junctions. These results suggest that JAC-1 acts as a positive modulator of cadherin function in C. elegans.

Two major isoforms of the cell adhesion molecule neurofascin NF186 and NF155 are expressed in the central nervous system (CNS). We have investigated their roles in the assembly of the node of Ranvier and show that they are targeted to distinct domains at the node. At the onset of myelination, NF186 is restricted to neurons, whereas NF155 localizes to oligodendrocytes, the myelin-forming glia of the CNS. Coincident with axon ensheathment, NF155 clusters at the paranodal regions of the myelin sheath where it localizes in apposition to the axonal adhesion molecule paranodin/contactin-associated protein (Caspr1), which is a constituent of the septate junction-like axo-glial adhesion zone. Immunoelectron microscopy confirmed that neurofascin is a glial component of the paranodal axo-glial junction. Concentration of NF155 with Caspr1 at the paranodal junctions of peripheral nerves is also a feature of Schwann cells. In Shiverer mutant mice, which assemble neither compact CNS myelin nor normal paranodes, NF155 (though largely retained at the cell body) is also distributed at ectopic sites along axons, where it colocalizes with Caspr1. Hence, NF155 is the first glial cell adhesion molecule to be identified in the paranodal axo-glial junction, where it likely interacts with axonal proteins in close association with Caspr1.

DdCAD-1 is a 24-kD Ca2+-dependent cell- cell adhesion molecule that is expressed soon after the initiation of development in Dictyostelium cells. DdCAD-1 is present on the cell surface as well as in the cytosol. However, the deduced amino acid sequence of DdCAD-1 lacks a hydrophobic signal peptide or any predicted transmembrane domain, suggesting that it may be presented on the cell surface via a nonclassical transport mechanism. Here we report that DdCAD-1 is transported to the cell surface via contractile vacuoles, which are normally involved in osmoregulation. Immunofluorescence microscopy and subcellular fractionation revealed a preferential association of DdCAD-1 with contractile vacuoles. Proteolytic treatment of isolated contractile vacuoles degraded vacuole-associated calmodulin but not DdCAD-1, demonstrating that DdCAD-1 was present in the lumen. The use of hyperosmotic conditions that suppress contractile vacuole activity led to a dramatic decrease in DdCAD-1 accumulation on the cell surface and the absence of cell cohesiveness. Shifting cells back to a hypotonic condition after hypertonic treatments induced a rapid increase in DdCAD-1-positive contractile vacuoles, followed by the accumulation of DdCAD-1 on the cell membrane. 7-chloro-4-nitrobenzo-2-oxa-1, 3-diazole, a specific inhibitor of vacuolar-type H+-ATPase and thus of the activity of contractile vacuoles, also inhibited the accumulation of DdCAD-1 on the cell surface. Furthermore, an in vitro reconstitution system was established, and isolated contractile vacuoles were shown to import soluble DdCAD-1 into their lumen in an ATP-stimulated manner. Taken together, these data provide the first evidence for a nonclassical protein transport mechanism that uses contractile vacuoles to target a soluble cytosolic protein to the cell surface.

The leukocyte adhesion molecule L-selectin mediates binding to lymph node high endothelial venules (HEV) and contributes to leukocyte rolling on endothelium at sites of inflammation. Previously, it was shown that truncation of the L-selectin cytoplasmic tail by 11 amino acids abolished binding to lymph node HEV and leukocyte rolling in vivo, but the molecular basis for that observation was not determined. This study examined potential interactions between L-selectin and cytoskeletal proteins. We found that the cytoplasmic domain of L-selectin interacts directly with the cytoplasmic actin-binding protein alpha-actinin and forms a complex with vinculin and possibly talin. Solid phase binding assays using the full-length L-selectin cytoplasmic domain bound to microtiter wells demonstrated direct, specific, and saturable binding of purified alpha-actinin to L-selectin (Kd = 550 nM), but no direct binding of purified talin or vinculin. Interestingly, talin potentiated binding of alpha-actinin to the L-selectin cytoplasmic domain peptide despite the fact that direct binding of talin to L-selectin could not be measured. Vinculin binding to the L-selectin cytoplasmic domain peptide was detectable only in the presence of alpha-actinin. L-selectin coprecipitated with a complex of cytoskeletal proteins including alpha-actinin and vinculin from cells transfected with L-selectin, consistent with the possibility that alpha-actinin binds directly to L-selectin and that vinculin associates by binding to alpha-actinin in vivo to link actin filaments to the L-selectin cytoplasmic domain. In contrast, a deletion mutant of L-selectin lacking the COOH-terminal 11 amino acids of the cytoplasmic domain failed to coprecipitate with alpha-actinin or vinculin. Surprisingly, this mutant L-selectin localized normally to the microvillar projections on the cell surface. These data suggest that the COOH-terminal 11 amino acids of the L-selectin cytoplasmic domain are required for mediating interactions with the actin cytoskeleton via a complex of alpha-actinin and vinculin, but that this portion of the cytoplasmic domain is not necessary for proper localization of L-selectin on the cell surface. Correct L-selectin receptor positioning is therefore insufficient for leukocyte adhesion mediated by L-selectin, suggesting that this adhesion may also require direct interactions with the cytoskeleton.

p120 was originally identified as a substrate of pp60src and several receptor tyrosine kinases, but its function is not known. Recent studies revealed that this protein shows homology to a group of proteins, beta-catenin/Armadillo and plakoglobin (gamma-catenin), which are associated with the cell adhesion molecules cadherins. In this study, we examined whether p120 is associated with E-cadherin using the human carcinoma cell line HT29, as well as other cell lines, which express both of these proteins. When proteins that copurified with E-cadherin were analyzed, not only alpha-catenin, beta-catenin, and plakoglobin but also p120 were detected. Conversely, immunoprecipitates of p120 contained E-cadherin and all the catenins, although a large subpopulation of p120 was not associated with E-cadherin. Analysis of these immunoprecipitates suggests that 20% or less of the extractable E-cadherin is associated with p120. When p120 immunoprecipitation was performed with cell lysates depleted of E-cadherin, beta-catenin was no longer coprecipitated, and the amount of plakoglobin copurified was greatly reduced. This finding suggests that there are various forms of p120 complexes, including p120/E-cadherin/beta-catenin and p120/E-cadherin/plakoglobin complexes; this association profile contrasts with the mutually exclusive association of beta-catenin and plakoglobin with cadherins. When the COOH-terminal catenin binding site was truncated from E-cadherin, not only beta-catenin but also p120 did not coprecipitate with this mutated E-cadherin. Immunocytological studies showed that p120 colocalized with E-cadherin at cell-cell contact sites, even after non-ionic detergent extraction. Treatment of cells with hepatocyte growth factor/scatter factor altered the level of tyrosine phosphorylation of p120 as well as of beta-catenin and plakoglobin. These results suggest that p120 associates with E-cadherin at its COOH-terminal region, but the mechanism for this association differs from that for the association of beta-catenin and plakoglobin with E-cadherin, and thus, that p120, whose function could be modulated by growth factors, may play a unique role in regulation of the cadherin-catenin adhesion system.

The integrity of the endothelial layer, which lines the entire cavity of the vascular system, depends on tight adhesion of the cells to the underlying basement membrane as well as to each other. It has been previously shown that such interactions occur via membrane receptors that determine the specificity, topology, and mechanical properties of the surface adhesion. Cell-cell junctions between endothelial cells, in culture and in situ, involve both Ca(2+)-dependent and -independent mechanisms that are mediated by distinct adhesion molecules. Ca(2+)-dependent cell-cell adhesion occurs mostly via members of the cadherin family, which locally anchor the microfilament system to the plasma membrane, in adherens junctions. Ca(2+)-independent adhesions were reported to mainly involve members of the Ig superfamily. In this study, we performed three-dimensional microscopic analysis of the relative subcellular distributions of these two endothelial intercellular adhesion systems. We show that cadherins are located at adjacent (usually more apical), yet clearly distinct domains of the lateral plasma membrane, compared to PECAM-1. Moreover, cadherins were first organized in adherens junctions within 2 h after seeding of endothelial cells, forming multiple lateral patches which developed into an extensive belt-like structure over a period of 24 h. PECAM-1 became associated with surface adhesions significantly later and became progressively associated with the cadherin-containing adhesions. Cadherins and PECAM-1 also differed in their detergent extractability, reflecting differences in their mode of association with the cytoskeleton. Moreover, the two adhesion systems could be differentially modulated since short treatment with the Ca2+ chelator EGTA, disrupted the cadherin junctions leaving PECAM-1 apparently intact. These results confirm that endothelial cells possess distinct intercellular contact mechanisms that differ in their spatial and temporal organization as well as in their functional properties.

Kalinin was purified from squamous cell carcinoma (SCC25) spent culture media using an immunoaffinity column prepared from the mAb BM165. The affinity-purified material was separated by SDS-PAGE into three bands of 165-155, 140, and 105 kD identical to those obtained from normal human keratinocyte cultures and previously identified as kalinin. Kalinin promoted adhesion of a large number of normal cells and established cell lines with an activity similar to other adhesion molecules such as the laminin-nidogen complex, fibronectin, or collagen IV. However, kalinin was a much better substrate than laminin-nidogen complex for adhesion of cells of epithelial origin including primary human keratinocytes. Adhesion to kalinin was followed by cell shape changes ranging from rounded to fully spread cells depending on the cell types. The adhesion-promoting activity of kalinin was conformation dependent and was abolished by heat denaturation. mAb BM165 prevented cell adhesion to kalinin but not to other extracellular matrix substrates. However, either complete or partial inhibition was observed with different cells suggesting the existence of at least two cell-binding sites on the kalinin molecule. Experiments inhibiting cell adhesion with function-blocking anti-integrin subunit antibodies indicated that both alpha 3 beta 1 and alpha 6 beta 1 integrins are involved in the cellular interactions with kalinin, while for cell adhesion to classical mouse Engelbreth-Holm-Swarm laminin only alpha 6 beta 1 integrins, and not alpha 3 beta 1, appeared to be functional. Altogether, these results suggest that kalinin may fulfill additional functions than laminin, particularly for epithelial cells.

The interaction of lymphocyte function-associated antigen-1 (LFA-1) with its ligands mediates multiple cell adhesion processes of capital importance during immune responses. We have obtained three anti-ICAM-3 mAbs which recognize two different epitopes (A and B) on the intercellular adhesion molecule-3 (ICAM-3) as demonstrated by sequential immunoprecipitation and cross-competitive mAb-binding experiments. Immunoaffinity purified ICAM-3-coated surfaces were able to support T lymphoblast attachment upon cell stimulation with both phorbol esters and cross-linked CD3, as well as by mAb engagement of the LFA-1 molecule with the activating anti-LFA-1 NKI-L16 mAb. T cell adhesion to purified ICAM-3 was completely inhibited by cell pretreatment with mAbs to the LFA-1 alpha (CD11a) or the LFA-beta (CD18) integrin chains. Anti-ICAM-3 mAbs specific for epitope A, but not those specific for epitope B, were able to trigger T lymphoblast homotypic aggregation. ICAM-3-mediated cell aggregation was dependent on the LFA-1/ICAM-1 pathway as demonstrated by blocking experiments with mAbs specific for the LFA-1 and ICAM-1 molecules. Furthermore, immunofluorescence studies on ICAM-3-induced cell aggregates revealed that both LFA-1 and ICAM-1 were mainly located at intercellular boundaries. ICAM-3 was located at cellular uropods, which in small aggregates appeared to be implicated in cell-cell contacts, whereas in large aggregates it appeared to be excluded from cell-cell contact areas. Experiments of T cell adhesion to a chimeric ICAM-1-Fc molecule revealed that the proaggregatory anti-ICAM-3 HP2/19 mAb was able to increase T lymphoblast attachment to ICAM-1, suggesting that T cell aggregation induced by this mAb could be mediated by increasing the avidity of LFA-1 for ICAM-1. Moreover, the HP2/19 mAb was costimulatory with anti-CD3 mAb for T lymphocyte proliferation, indicating that enhancement of T cell activation could be involved in ICAM-3-mediated adhesive phenomena. Altogether, our results indicate that ICAM-3 has a regulatory role on the LFA-1/ICAM-1 pathway of intercellular adhesion.

We have previously shown that peptides derived from the thrombospondin sequence, CSVTCG, promoted tumor cell adhesion. To further investigate this observation, the CSVTCG-tumor cell adhesion receptor from A549 human lung adenocarcinoma cells was isolated and characterized. A single protein peak was isolated by CSVTCG affinity chromatography which also analyzed as a single peak by anion exchange chromatography. The purified protein had a pI of 4.7 and analyzed on SDS-gels as a single band of M(r) = 50,000 under nonreducing conditions and as two protein bands of M(r) = 50,000, and 60,000 under reducing conditions. Purified CSVTCG binding protein (CBP) bound either CSVTCG- or TSP-Sepharose but showed little interaction with either VCTGSC- or BSA-Sepharose. CBP was cell surface exposed. CSVTCG derivatized with [125I] Bolton-Hunter reagent was taken up by cells in a dose-dependent manner and the cell association was inhibited with a monospecific polyclonal anti-CBP antibody. Examination of the cell proteins crosslinked to labeled CSVTCG by SDS-gel electrophoresis revealed one band that comigrated with purified CPB. Using an in vitro binding assay, purified CBP bound mannose, galactose, and glucosamine-specific lectins. CBP bound TSP saturably and reversibly. The binding was Ca+2/Mg+2 ion dependent and inhibited with fluid phase TSP and anti-CBP. Little or no binding was observed on BSA, fibronectin, GRGES, and GRGDS. Heparin, but not lactose, inhibited binding. Anti-CBP IgG and anti-CSVTCG peptide IgG inhibited A549 cell spreading and adhesion on TSP but not on fibronectin and laminin. These results indicate that CBP and the CSVTCG peptide domain of TSP can mediate TSP-promoted tumor cell adhesion.

Tyrosine phosphorylation of membrane-associated proteins is involved at two distinct sites of contact between cells and the extracellular matrix: adhesion plaques (cell adhesion and de-adhesion) and invadopodia (invasion into the extracellular matrix). Adhesion plaques from chicken embryonic fibroblasts or from cells transformed by Rous sarcoma virus contain low levels of tyrosine-phosphorylated proteins (YPPs) which were below the level of detection in 0.5-microns thin, frozen sections. In contrast, intense localization of YPPs was observed at invadopodia of transformed cells at sites of degradation and invasion into the fibronectin-coated gelatin substratum, but not in membrane extensions free of contact with the extracellular matrix. Local extracellular matrix degradation and formation of invadopodia were blocked by genistein, an inhibitor of tyrosine-specific kinases, but cells remained attached to the substratum and retained their free-membrane extensions. Invadopodia reduced or lost YPP labeling after treatment of the cells with genistein, but adhesion plaques retained YPP labeling. The plasma membrane contact fractions of normal and transformed cells have been isolated form cells grown on gelatin cross-linked substratum using a novel fractionation scheme, and analyzed by immunoblotting. Four major YPPs (150, 130, 81, and 77 kD) characterize invadopodial membranes in contact with the matrix, and are probably responsible for the intense YPP labeling associated with invadopodia extending into sites of matrix degradation. YPP150 may be an invadopodal-specific YPP since it is approximately 3.6-fold enriched in the invasive contact fraction relative to the cell body fraction and is not observed in normal contacts. YPP130 is enriched in transformed cell contacts but may also be present in normal contacts. The two major YPPs of normal contacts (130 and 71 kD) are much lower in abundance than the major tyrosine-phosphorylated bands associated with invadopodial membranes, and likely represent major adhesion plaque YPPs. YPP150, paxillin, and tensin appear to be enriched in the cell contact fractions containing adhesion plaques and invadopodia relative to the cell body fraction, but are also present in the soluble supernate fraction. However, vinculin, talin, and alpha-actinin that are localized at invadopodia, are equally concentrated in cell bodies and cell contacts as is the membrane-adhesion receptor beta 1 integrin. Thus, tyrosine phosphorylation of the membrane-bound proteins may contribute to the cytoskeletal and plasma membrane events leading to the formation and function of invadopodia that contact and proteolytically degrade the extracellular matrix; we have identified several candidate YPPs that may participate in the regulation of these processes.

A major polypeptide of M(r) 37,000 was purified from a desmosome-enriched citric acid-insoluble pellet of pig tongue epithelium. The polypeptide was solubilized from the 4-M urea-insoluble pellet with 9 M urea, and extracts were separated by carboxymethyl cellulose and gel filtration chromatography. The 37-kD protein was obtained in milligram quantities as a single band on two-dimensional gels in 30% yield after 21-fold purification from the citric acid-insoluble fraction. The protein is not glycosylated and has a pI of approximately 8.7. Although isolated from a fraction rich in desmosomes, the 37-kD protein is not a desmosomal protein. Indirect immunofluorescence analysis of frozen sections of tongue and other tissues demonstrated that antibodies raised to the 37-kD protein bound only to suprabasal cell layers at punctate regions of the periphery of the cell and was absent from most regions of epidermis, whereas antibodies to desmoplakins I and II, desmosomal proteins, bound similarly but in all epidermal layers. Immunoelectron microscopy localized the 37-kD protein to the cell periphery in regions between, but never in, desmosomes. By immunofluorescence, the 37-kD protein colocalized with actin as well as with vinculin and uvomorulin in oral tissues. Like the 37-kD protein, vinculin and uvomorulin were absent from the basal layer. Based on its appearance, localization, and solubility properties, the 37-kD protein is probably a component of adherens junctions; its restriction to suprabasal cells and exclusion from the epidermis are unique.

Hyalin is a large (ca. 350 x 10(3) kD by gel electrophoresis) molecule that contributes to the hyalin layer surrounding the sea urchin embryo. In previous work a mAb (McA Tg-HYL), specific for hyalin, was found to inhibit cell-hyalin adhesion and block morphogenesis of whole embryos (Adelson, D. L., and T. D. Humphreys. 1988. Development. 104:391-402). In this report, hyalin ultrastructure was examined via rotary shadowing. Hyalin appeared to be a filamentous molecule approximately 75-nm long with a globular "head" about 12 nm in diameter that tended to form aggregates by associating head to head. Hyalin molecules tended to associate with a distinct high molecular weight globular particle ("core"). In fractions containing the core particle often more than one hyalin molecule were seen to be associated with the core. The core particle maintained a tenacious association with hyalin throughout purification procedures. The site(s) of McA Tg-HYL binding to the hyalin molecule were visualized by decorating purified hyalin with the antibody and then rotary shadowing the complex. In these experiments, McA Tg-HYL attached to the hyalin filament near the head region in a pattern suggesting that more than one antibody binding site exists on the hyalin filament. From the ultrastructural data and from the cell adhesion data presented earlier we conclude that hyalin is a filamentous molecule that binds to other hyalin molecules and contains multiple cell binding sites. Attempts were made to demonstrate the existence of lower molecular weight hyalin precursors. Whilst no such precursors could be identified by immunoprecipitation of in vivo labeled embryo lysates, immunoprecipitation of in vitro translation products suggested such precursors (ca 40 x 10(3) kD) might exist.

Basal keratinocytes attach to the underlying dermal stroma through an ultrastructurally unique and complex basement membrane zone. Electron-dense plaques along the basal surface plasma membrane, termed hemidesmosomes, appear to attach directly to the lamina densa of the basement membrane through fine strands, called anchoring filaments. The lamina densa is secured to the stroma through a complex of type VII collagen containing anchoring fibrils and anchoring plaques. We have identified what we believe is a novel antigen unique to this tissue region. The mAbs to this antigen localize to the anchoring filaments, just below the basal-dense plate of the hemidesmosomes. In cell culture, the antigen is deposited upon the culture substate by growing and migrating human keratinocytes. Addition of mAb to the cultures causes the cells to round and detach, but does not impair them metabolically. Skin fragments incubated with antibody extensively de-epithelialize. These findings strongly suggest that this antigen is intimately involved in attachment of keratinocytes to the basement membrane. This antigen was isolated from keratinocyte cultures by immunoaffinity chromatography. Two molecules are observed. The most intact species contains three nonidentical chains, 165, 155, and 140 kD linked by interchain disulfide bonds. The second and more abundant species contains the 165- and 140-kD chains, but the 155-kD chain has been proteolytically cleaved to 105 kD. Likewise, two rotary-shadowed images are observed. The larger of the two, presumably corresponding to the most intact form, appears as an asymmetric 107-nm-long rod, with a single globule at one end and two smaller globules at the other. The more abundant species, presumably the proteolytically cleaved form, lacks the distal small globule. We propose the name "kalinin" for this new molecule.

Lymphocyte attachment to high endothelial venules within lymph nodes is mediated by the peripheral lymph node homing receptor (pnHR), originally defined on mouse lymphocytes by the MEL-14 mAb. The pnHR is a calcium-dependent lectin-like receptor, a member of the LEC-CAM family of adhesion proteins. Here, using a soluble recombinant form of the homing receptor, we have identified an endothelial ligand for the pnHR as an approximately 50-kD sulfated, fucosylated, and sialylated glycoprotein, which we designate Sgp50 (sulfated glycoprotein of 50 kD). Recombinant receptor binding to this lymph node-specific glycoprotein requires calcium and is inhibitable by specific carbohydrates and by MEL-14 mAb. Sialylation of the component is required for binding. Additionally, the glycoprotein is precipitated by MECA-79, an adhesion-blocking mAb reactive with lymph node HEV. A related glycoprotein of approximately 90 kD (designated as Sgp90) is also identified.

It has recently become clear that both extracellular matrix (ECM) glycoproteins and various cell adhesion molecules (CAMs) can promote neurite outgrowth from primary neurons, though little is known of the intracellular mechanisms through which these signals are transduced. We have previously obtained evidence that protein kinase C function is an important part of the neuronal response to laminin (Bixby, J.L. 1989. Neuron. 3:287-297). Because such CAMs as L1 (Lagenauer, C., and V. Lemmon. 1987. Proc. Natl. Acad. Sci. USA. 84:7753-7757) and N-cadherin (Bixby, J.L. and R. Zhang. 1990. J. Cell Biol. 110:1253-1260) can be purified and used as substrates to promote neurite growth, we have now tested whether the response to CAMs is similarly dependent on protein kinase C. We find that inhibition of protein kinase C inhibits growth on fibronectin or collagen as well as on laminin. In contrast, C kinase inhibition actually potentiates the initial growth response to L1 or N-cadherin. The later "phase" of outgrowth on both of these CAMs is inhibited, however. Additionally, phorbol esters, which have no effect on neurite growth when optimal laminin concentrations are used, potentiate growth even on optimal concentrations of L1 or N-cadherin. The results indicate that different intracellular mechanisms operate during initial process outgrowth on ECM substrates as compared to CAM substrates, and suggest that protein kinase C function is required for continued neurite growth on each of these glycoproteins.

The 68-kD protein (paxillin) is a cytoskeletal component that localizes to the focal adhesions at the ends of actin stress fibers in chicken embryo fibroblasts. It is also present in the focal adhesions of Madin-Darby bovine kidney (MDBK) epithelial cells but is absent, like talin, from the cell-cell adherens junctions of these cells. Paxillin purified from chicken gizzard smooth muscle migrates as a diffuse band on SDS-PAGE gels with a molecular mass of 65-70 kD. It is a protein of multiple isoforms with pIs ranging from 6.31 to 6.85. Using purified paxillin, we have demonstrated a specific interaction in vitro with another focal adhesion protein, vinculin. Cleavage of vinculin with Staphylococcus aureus V8 protease results in the generation of two fragments of approximately 85 and 27 kD. Unlike talin, which binds to the large vinculin fragment, paxillin was found to bind to the small vinculin fragment, which represents the rod domain of the molecule. Together with the previous observation that paxillin is a major substrate of pp60src in Rous sarcoma virus-transformed cells (Glenney, J. R., and L. Zokas. 1989. J. Cell Biol. 108:2401-2408), this interaction with vinculin suggests paxillin may be a key component in the control of focal adhesion organization.

Cell-cell adhesion is controlled by many molecules found on the cell surface. In addition to the constituents of well-defined junctional structures, there are the molecules that are thought to play a role in the initial interactions of cells and that appear at precise times during development. These include the cadherins and cell adhesion molecules (CAMs). Representatives of these families of adhesion molecules have been isolated from most of the major tissues. The notable exception is the vascular endothelium. Here we report the identification of a cell surface molecule designated "endoCAM" (endothelial Cell Adhesion Molecule), which may function as an endothelial cell-cell adhesion molecule. EndoCAM is a 130-kD glycoprotein expressed on the surface of endothelial cells both in culture and in situ. It is localized to the borders of contiguous endothelial cells. It is also present on platelets and white blood cells. Antibodies against endoCAM prevent the initial formation of endothelial cell-cell contacts. Despite similarities in size and intercellular location, endoCAM does not appear to be a member of the cadherin family of adhesion receptors. The serologic and protease susceptibility characteristics of endoCAM are different from those of the known cadherins, including an endogenous endothelial cadherin. Although the precise biologic function of endoCAM has not been determined, it appears to be one of the molecules responsible for regulating endothelial cell-cell adhesion processes and may be involved in platelet and white blood cell interactions with the endothelium.