An activated Ras protein alters cell adhesion by dephosphorylating Dictyostelium DdCAD-1

Abstract

RasG-regulated signal transduction has been linked to a variety of growth-specific processes and appears to also play a role in the early development of Dictyostelium discoideum. In an attempt to uncover some of the molecular components involved in Ras-mediated signalling, several proteins have been described previously, including the cell adhesion molecule DdCAD-1, whose phosphorylation state was affected by the expression of the constitutively activated RasG, RasG(G12T). Here it has been shown that a cadA null strain lacks the phosphoproteins that were tentatively identified as DdCAD-1, confirming its previous designation. Further investigation revealed that cells expressing RasG(G12T) exhibited increased cell-cell cohesion, concomitant with reduced levels of DdCAD-1 phosphorylation. This increased cohesion was DdCAD-1-dependent and was correlated with increased localization of DdCAD-1 at the cell surface. DdCAD-1 phosphorylation was also found to decrease during Dictyostelium aggregation. These results revealed a possible role for protein phosphorylation in regulating DdCAD-1-mediated cell adhesion during early development. In addition, the levels of DdCAD-1 protein were substantially reduced in a rasG null cell line. These results indicate that RasG affects both the expression and dephosphorylation of DdCAD-1 during early development.

Dictyostelium discoideum Paxillin Regulates Actin-Based Processes

“…It has been shown that cell agglomerates form when cells are washed free of growth medium, resuspended in non-nutrient buffer, and allowed to develop in suspension (Secko et al 2006). We found that wild-type cells formed a few small agglomerates but, for the most part, remained as single cells after three hours of starvation (Fig.…”

“…It is important for initial stream formation, creation of the mound, morphogenesis, and cell type differentiation (Siu et al 2004). Aggregating cells differ from growth-phase cells by their increased cohesion (Secko et al 2006). The overexpression of paxB increases cell-cell cohesion, indicating that paxB plays a positive role in cell-cell cohesion.…”

“…One putative PaxB interacting protein is the D. discoideum homologue of mammalian cadherin, DdCAD-1. DdCAD-1 is a Ca 2+ dependent adhesion molecule, and like PaxB, is enriched in regions of cell-cell contact, is involved in cell-cell cohesion and influences cell sorting (Desbarats et al 1994; Lin et al 2006; Secko et al 2006; Sesaki and Siu 1996; Wong et al 2002; Yang 1997). To date, a direct interaction between paxillin and cadherins has not been examined in any organism.…”

“…Dictyostelium growth phase cells are barely cohesive, forming transient contacts or loose aggregates, which are easily dissociated with 10 mM EDTA or EGTA (Secko et al 2006; Yuan et al 2001). Aggregating cells differ from growth-phase cells by their stronger cohesion, which leads to formation of large EDTA-resistant aggregates in shaken suspension (Kamboj et al 1989; Siu et al 2004).…”

Diverse evolutionary paths to cell adhesion

“…In animals, the phosphorylation state of an adhesion protein often has dramatic affects on its adhesive properties [51]. Similarly, ras-dependent dephosphorylation of DdCAD-1 increases DdCAD-1-mediated adhesion of cells [52]. …”