Functional interaction between the SSeCKS scaffolding protein and the cytoplasmic domain of β1,4-galactosyltransferase

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Abstract
The β1,4-galactosyltransferase family contains at least seven unique gene products, of which β1,4-galactosyltransferase I (GalT) is the most exhaustively studied. GalT exists in the Golgi complex, similar to many other glycosyltransferases, as well as on the cell surface, where it functions as a signaling receptor for extracellular glycoside ligands. When expressed on the surface, GalT associates with the cytoskeleton and, upon ligand-induced aggregation, induces cell-type specific intracellular signal cascades. In an effort to define the mechanisms by which surface GalT exerts these intracellular effects, we used the yeast two-hybrid system to identify proteins that specifically interact with the GalT cytoplasmic domain. The yeast two-hybrid screen identified two distinct clones (1.12 and 2.52) that showed identity to portions of SSeCKS (Src Suppressed CKinase Substrate). SSeCKS is a previously defined kinase and cytoskeleton scaffolding protein whose subcellular distribution and functions are remarkably similar to those attributed to GalT. Both SSeCKS and GalT have been localized to the perinuclear/Golgi region as well as to filopodia/lamellipodia. SSeCKS and GalT have been implicated in regulating cell growth, actin filament dynamics, and cell spreading. Interestingly, 1.12 and 2.52-GFP constructs were localized to subcellular domains that correlated with the two purported subcellular distributions for GalT; 2.52 being confined to the Golgi, whereas 1.12 localized primarily to filopodia. Coimmunoprecipitation assays demonstrate stable binding between the GalT cytoplasmic domain and the 1.12 and 2.52 domains of SSeCKS in appropriately transfected cells. Similar assays demonstrate binding between the endogenous GalT and SSeCKS proteins also. Coimmunoprecipitation assays were performed in both directions and produced similar results (i.e. using either anti-GalT domain or anti-SSeCKS domain antibodies as the precipitating reagent). A functional interaction between the GalT cytoplasmic domain and SSeCKS was illustrated by the ability of either the 1.12 or 2.52 SSeCKS domain to restore a normal adhesive phenotype in cells overexpressing the TL-GFP dominant negative construct. TL-GFP is composed of the GalT cytoplasmic and transmembrane domains fused to GFP, and leads to a loss of cell adhesion on laminin by displacing the endogenous GalT from its cytoskeleton binding sites. This is the first reported interaction between a glycosyltransferase and a scaffolding protein, and suggests that SSeCKS serve to integrate the various functions ascribed to the GalT cytoplasmic domain.