Abstract. The rho proteins, p21 ~, are ubiquitously expressed guanine nucleotide binding proteins with ,'o30% amino acid homology to p21'% but their biochemical function is unknown. We show here that microinjection of constitutively activated recombinant rho protein (Vall4rho) into subconfluent cells induces dramatic changes in cell morphology: 15-30 min after injection cells adopt a distinct and novel phenotype with a contracted cell body and finger-like processes still adherent to the substratum. Ribosylation of Vall4rho with the ADP-ribosyltransferase C3 from clostridium botulinum, before microinjection, renders the protein biologically inactive, but it has no effect on either its intrinsic biochemical properties or on its interaction with the GTPase activating protein, rho GAP. Microinjection of ribosylated normal rho, on the other hand, has a similar effect to injection of C3 transferase and induces complete rounding up of cells. We also report striking biochemical changes in actin filament organization when contact-inhibited quiescent 3T3 cells are injected with Vall4rho protein. The effects induced by activation or inactivation of p21 ~ described here, suggest that the biological function of this protein is to control some aspect of cytoskeletal organization.

PTEN is a tumor suppressor protein that dephosphorylates phosphatidylinositol 3,4,5 trisphosphate and antagonizes the phosphatidylinositol-3 kinase signaling pathway. We show here that PTEN can also inhibit cell migration through its C2 domain, independent of its lipid phosphatase activity. This activity depends on the protein phosphatase activity of PTEN and on dephosphorylation at a single residue, threonine(383). The ability of PTEN to control cell migration through its C2 domain is likely to be an important feature of its tumor suppressor activity.