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The dynamic rearrangement of cell-cell adhesion is one of the major physiological events in tissue development and tumor metastasis. Polarized ceil migration, another key event, is a tightly regulated process that occurs during tissue development, chemotaxis and wound healing. Rho-family small GTPases, especially Rac1 and Cdc42, play pivotal roles in these processes through one of their effectors, IQGAPI. Recent studies reveal that IQGAPI regulates cadherin-mediated cell-cell adhesion both positively and negatively. It captures and stabilizes microtubules through the microtubule-binding protein CLIP-170 near the cell cortex, leading to establishment of polarized cell morphology and directional cell migration. Furthermore, Rac1 and Cdc42 link the adenomatous polyposis coli (APC) protein to actin filaments through IQGAP1 at the leading edge and thereby regulate polarization and directional migration. [ABSTRACT FROM AUTHOR]

Polarised cell migration is required for various cell behaviours and functions. Actin and microtubules are coupled structurally and distributed asymmetrically along the front-rear axis of migrating cells. CLIP-associating proteins (CLASPs) accumulate near the ends of microtubules at the front of migrating cells to control microtubule dynamics and cytoskeletal coupling. Regional inhibition of GSK-3β is responsible for this asymmetric distribution of CLASPs. However, it is not known how GSK-3β regulates the activity of CLASPs for linkage between actin and microtubules. Here we identified IQGAP1, an actin-binding protein, as a novel CLASP-binding protein. GSK-3β directly phosphorylates CLASP2 at Ser533 and Ser537 within the region responsible for the IQGAP1 binding. Phosphorylation of CLASP2 results in the dissociation of CLASP2 from IQGAP1, EB1 and microtubules. At the leading edges of migrating fibroblasts, CLASP2 near microtubule ends partially colocalises with IQGAP1. Expression of active GSK-3β abrogates the distribution of CLASP2 on microtubules, but not that of a nonphosphorylatable CLASP2 mutant. The phosphorylated CLASP2 does not accumulate near the ends of microtubules at the leading edges. Thus, phosphorylation of CLASP2 by GSK-3β appears to control the regional linkage of microtubules to actin filaments through IQGAP1 for cell migration. [ABSTRACT FROM AUTHOR]

Rac1 and Cdc42, members of the Rho family GTPases, control diverse cellular processes such as cell migration and morphogenesis through their effectors. Among the effectors, IQGAP1 plays pivotal roles in the establishment of cytoskeletal architecture and intercellular adhesions in various cells. However, its roles remain to be clarified, especially in neuronal cells. We have identified IQGAP3 as a novel member of the IQGAP family, which is highly expressed in brain. We found that IQGAP3, an effector of Rac1 and Cdc42, associates directly with actin filaments and accumulates asymmetrically at the distal region of axons in hippocampal neurons. The depletion of IQGAP3 impairs neurite or axon outgrowth in neuronal cells with the disorganized cytoskeleton, but depletion of IQGAP1 does not. Furthermore, IQGAP3 is indispensable for Rac1/Cdc42-promoted neurite outgrowth in PC12 cells. Taken together, these results indicate that IQGAP3 can link the activation of Rac1 and Cdc42 with the cytoskeletal architectures during neuronal morphogenesis. [ABSTRACT FROM AUTHOR]