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Mutations in the adenomatous polyposis coli (APC) gene are associated with familial adenomatous polyposis and sporadic colorectal tumours. The APC gene is expressed ubiquitously in various tissues, especially throughout the large intestine and central nervous system (CNS). In the CNS, the expression of the APC protein is highest during embryonic and early postnatal development. APC associates through its C-terminal region with postsynaptic density (PSD)-95, a neuronal protein that participates in synapse development. Here, we examined the involvement of APC in synaptogenesis. In cultured hippocampal neurons, both overexpression of a dominant-negative construct that disrupts the APC–PSD-95 interaction and knockdown of APC expression using small interfering RNA (siRNA) inhibited the clustering of PSD-95 and a glutamate receptor subunit, and reduced alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA)-induced activity of AMPA receptors; however, the clustering of an N-methyl-d-aspartate (NMDA) receptor subunit was unaffected. These results are suggestive of APC involvement in the development of glutamatergic synapses. [ABSTRACT FROM AUTHOR]

Abstract Background: hDLG, the human homologue of the Drosophila tumour suppressor dlg , functions as a scaffolding protein that facilitates the transmission of diverse downstream signals. hDLG possesses multiple protein-binding domains, including three PDZ domains, an SH3 domain, a HOOK domain and a guanylate kinase-like (GK) domain. Results: We studied the significance of the PDZ, SH3, HOOK and GK domains in the cytoplasmic localization of hDLG. We found that mutation of the SH3 or GK domain, but not the PDZ domain, resulted in a re-localization of hDLG to the nucleus. Furthermore, hDLG was found to possess a potential nuclear localization signal in the HOOK domain. Conclusion: These results suggest that the SH3, HOOK and GK domains of hDLG are important for its cytoplasmic localization. [ABSTRACT FROM AUTHOR]

The tumour suppressor gene adenomatous polyposis coli (APC) is mutated in sporadic and familial colorectal tumours. APC is involved in the proteasome-mediated degradation of β-catenin, through its interaction with β-catenin, GSK-3β and Axin. APC also interacts with the microtubule cytoskeleton and has been localized to clusters near the distal ends of microtubules at the edges of migrating epithelial cells. Moreover, in Xenopus laevis epithelial cells, APC has been shown to move along microtubules and accumulate at their growing plus ends. However, the mechanism of APC accumulation and the nature of these APC clusters remain unknown. We show here that APC interacts with the kinesin superfamily (KIF) 3A?KIF3B proteins, microtubule plus-end-directed motor proteins, through an association with the kinesin superfamily-associated protein 3 (KAP3). The interaction of APC with KAP3 was required for its accumulation in clusters, and mutant APCs derived from cancer cells were unable to accumulate efficiently in clusters. These results suggest that APC and β-catenin are transported along microtubules by KAP3?KIF3A?KIF3B, accumulate in the tips of membrane protrusions, and may thus regulate cell migration. [ABSTRACT FROM AUTHOR]

Background The synaptic protein PSD-95/SAP90 interacts with ion channels such as the N-methyl-D-aspartate-receptor (NMDA-R) via its PDZ domain, and is involved in their clustering. Moreover, it interacts with signalling molecules and plays an important role in coupling NMDA-R to pathways that control synaptic plasticity and learning. Results We report that PSD-95 interacts with the adenomatous polyposis coli (APC) tumour suppressor protein via its PDZ domain. Furthermore, we found that PSD-95, NMDA-R and APC are contained in the same complex in vivo. PSD-95-NMDA-R–APC association was found to require two cysteine residues conserved in the amino-terminus of PSD-95 that are known to be critical for its multimerization. Conclusion Our findings suggest that the PSD-95-NMDA-R-APC complex forms due to the multimerization of PSD-95 monomers, each of which can associate with either NMDA-R or APC. It is possible that APC is involved in the regulation of ion channel clustering and/or organization of signalling molecules. [ABSTRACT FROM AUTHOR]

HTLV-1 Tax oncoprotein interacts with various cellular factors and modulates transcription and the cell cycle. To identify more cellular targets, we employed the yeast two hybrid system with Tax using a human cDNA library, and isolated a cDNA encoding the human counterpart of Drosophila discs large tumor suppressor protein, hDLG. Tax binding to hDLG was confirmed in vitro and also in HTLV-1-infected T-cells. Furthermore, hDLG was found to be efficiently phosphorylated in Tax-transfected cells and HTLV-1-infected T-cells. The C-terminus of Tax and the PDZ domain of hDLG were responsible for the binding of Tax to hDLG. The C-terminal peptide of Tax prevented the binding of hDLG to APC tumor suppressor gene product, suggesting inhibition of hDLG function by Tax. Over-expression of hDLG in NIH3T3 cells by microinjection induced a reduction of BrdU incorporation into DNA, but co-expression of Tax suppressed this inhibitory effect of hDLG. These results suggest that hDLG arrested the cell cycle and that Tax canceled this inhibitory action of hDLG through targeting hDLG. Therefore, Tax affects this novel regulatory pathway of the cell cycle alteration, of which seems to play a role in the development of human cancer. [ABSTRACT FROM AUTHOR]

Abstract The human homologue of Drosophila tumor suppressor dlg, hDLG1, is one of the proteins known to interact with APC, a tumor suppressor for colorectal cancer. Alternative splicing of this gene generates transcripts either with [insertion 1 (I1)] or without 99 nucleotides in the 5' part of the dlg homology repeats (DHR) domain. We found almost equivalent expression of these two splicing variants in most human tissues; however, in skeletal muscle the transcript with the 99-bp insertion was predominant, and in the brain, that without the 99-bp insertion was expressed predominantly. We also examined alternative splicing in the region between the SH3 and GUK domains where two different sizes of insertions, 34 nucleotides (I2) or 100 nucleotides (I3), had been reported, and found various splicing patterns among the tissues examined. In brain we detected six different, alternatively spliced transcripts, two of which included a novel, 36-bp, brain-specific exon encoding a peptide bearing significant homology to a portion of rat synapse-associated protein, SAP97/PSD95. Subsequently, we investigated the splicing patterns of the hDLG1 gene in 24 neuroblastoma cell lines. In two-thirds of these lines, the splicing patterns were altered from those observed in normal brain tissue. As one-third retained the normal brain-splicing pattern, the loss of normal splicing of hDLG1 may not in itself cause formation of tumors, but it might reflect the biological character of individual neuroblastomas. [ABSTRACT FROM AUTHOR]

Dlgh1 (discs large homolog 1) is a mammalian homolog of the Drosophila tumor suppressor Discs large 1, and is a member of the membrane-associated guanylate kinase (MAGUK) scaffolding proteins that contain three PSD-95/Dlg/ZO-1 (PDZ) domains. Discs large 1 is involved in epithelial polarization and cell-cell adhesion complex formation during Drosophila development. However, the functions of Dlgh1 during mammalian development remain to be elucidated. We generated Dlgh1-knockout mice and found that homozygous Dlgh1-knockout mice developed various abnormalities in their renal and urogenital organs. The kidneys and ureters were hypoplastic and the lower ends of the ureters were ectopic. In addition, the vagina and seminal vesicle, which are derived from the lower part of the Müllerian and Wolffian duct, respectively, were absent. Unexpectedly, loss of Dlgh1 function in the developing ureters did not disrupt cell-cell junctional complexes, but did impair cellular proliferation in the epithelium. These results suggest a novel role for Dlgh1 in regulating epithelial duct formation and morphogenesis during mammalian development. Although congenital absence of the vagina associated with other variable Müllerian duct abnormalities has been reported in humans, its mechanism has not yet been clarified. Our findings might contribute to a better understanding of such abnormalities. [ABSTRACT FROM AUTHOR]

The human homolog of the Drosophila discs large tumor suppressor protein (hDLG) functions as a scaffolding protein that facilitates the transmission of diverse downstream signals. Here we show that hDLG interacts through its PDZ domains with the carboxy-terminal S/TXV motif of the mitochondrial ribosomal protein S-34 (MRP-S34). Our results suggest that hDLG interacts with MRP-S34 prior to entry of MRP-S34 into the mitochondria and may be involved in the trafficking of MRP-S34. [Copyright &y& Elsevier]