Involvement of a Rab8-like protein of Dictyostelium discoideum, Sas1, in the formation of membrane extensions, secretion and adhesion during development.

Establishment of cell-cell adhesions, regulation of actin, and secretion are critical during development. Rab8-like GTPases have been shown to modulate these cellular events, suggesting an involvement in developmental processes. To further elucidate the function of Rab8-like GTPases in a developmental context, a Rab8-related protein (Sas1) of Dictyostelium discoideum was examined, the expression of which increases at the onset of development. Dictyostelium cell lines expressing inactive (N128I mutant) and constitutively active (Q74L mutant) Sas1 as green fluorescent protein (GFP)-Sas1 chimeras were generated. Cells expressing Sas1Q74L displayed numerous actin-rich membrane protrusions, increased secretion, and were unable to complete development. In particular, these cells demonstrated a reduction in adhesion as well as in the levels of a cell adhesion molecule, gp24 (DdCAD-1). In contrast, cells expressing Sas1N128I exhibited increased cell-cell adhesion and increased levels of gp24. Counting factor is a multisubunit signalling complex that is secreted in early development and controls aggregate size by negatively regulating the levels of cell adhesion molecules, including gp24. Interestingly, the Sas1Q74L mutant demonstrated increased levels of extracellular countin, a subunit of counting factor, suggesting that Sas1 may regulate trafficking of counting factor components. Together, the data suggest that Sas1 may be a key regulator of actin, adhesion and secretion during development.