No abstract

A stereotyped, three-dimensional network of cell-cell contacts mediated by adherens junctions and cell-extracellular matrix contacts mediated by focal adhesions defines the architecture of the Drosophila ommatidium. Developmental reconstruction shows that this network is built in an incremental and generally conservative sequence; contacts established early in eye development typically persist into adulthood. Reconstructions show that photoreceptor apical surfaces are involuted into the retinal epithelium and are subsequently elaborated to form the photosensitive rhabdomeres. Rhabdomeres become aligned to the ommatidial optical axis via their anchorage to the retinal floor at the cone cell plate, a specialized nexus of cell-cell and cell-extracellular matrix contacts. Parallel reconstructions of retinal development in integrin mutants show that several eye phenotypes trace their origin to the structural failure of the cone cell plate.

Syndecan-4 is a transmembrane heparan sulfate proteoglycan that acts as a coreceptor with integrins in focal adhesion formation. The central region of syndecan-4 cytoplasmic domain (4V; LGKKPIYKK) binds phosphatidylinositol 4,5-bisphosphate, and together they regulate protein kinase C alpha (PKC alpha) activity. Syndecan 4V peptide directly potentiates PKC alpha activity, leading to "superactivation" of the enzyme, apparently through an interaction with its catalytic domain. We now have performed yeast two-hybrid and in vitro binding assays to determine the interaction sites between 4V and PKC alpha. Full-length PKC alpha weakly interacted with 4V by yeast two-hybrid assays, but PKC alpha constructs that lack the pseudosubstrate region or constructs of the whole catalytic domain interacted more strongly. A mutated 4V sequence (4V(YF): LGKKPIFKK) did not interact with PKC alpha, indicating that tyrosine 192 in the syndecan-4 cytoplasmic domain might be critical for this interaction. Further assays identified a novel interaction site in the C terminus of the catalytic domain of PKC alpha (amino acid sequence 513-672). This encompasses the autophosphorylation sites, which are implicated in activation and stability. Yeast two-hybrid data were confirmed by in vitro binding and coimmunoprecipitation assays. The interaction of syndecan-4 with PKC alpha appears unique since PKC delta and epsilon did not interact with 4V in yeast two-hybrid assays or coimmunoprecipitate with syndecan-4. Finally, overexpression of syndecan-4 in rat embryo fibroblast cells, but not expression of the YF mutant, increased PKC alpha localization to focal adhesions. The data support a mechanism where syndecan-4 binds PKC alpha and localizes it to focal adhesions, whose assembly may be regulated by the kinase.

Syndecan-4 is a widely expressed transmembrane heparan sulfate proteoglycan which localizes to focal adhesions. Previous studies showed that the syndecan-4 cytoplasmic domain can associate with and potentiate the activity of protein kinase C, which is required for focal adhesion formation. To examine further the role of syndecan-4 in cell adhesion, we expressed syndecan-4 cDNA constructs in CHO-K1 cells. Syndecan-2 transfection was used to confirm effects seen were specific for syndecan-4. Cells overexpressing full length syndecan-4 core protein exhibited a more flattened, fibroblastic morphology, with increased focal adhesion formation and decreased cell motility. Expression of a syndecan-4 core protein with either a partial or complete deletion of the cytoplasmic domain or of an antisense construct led to markedly decreased spreading and focal adhesion formation, a more epithelioid morphology, and decreased motility. Overexpression of syndecan-2 changed the adhesive phenotype, but did not markedly alter focal adhesion and microfilament bundle formation. The data suggest that syndecan-4 is a regulator of focal adhesion and stress fiber formation, and influences both morphology and migration.