RAB8 PROMOTES POLARIZED MEMBRANE-TRANSPORT THROUGH REORGANIZATION OF ACTIN AND MICROTUBULES IN FIBROBLASTS

Rab8 is a small Ras-like GTPase that regulates polarized membrane tran sport to the basolateral membrane in epithelial cells and to the dendr ites in neurons. It has recently been demonstrated that fibroblasts so rt newly synthesized proteins into two different pathways for delivery to the cell surface that are equivalent to the apical and the basolat eral post-Golgi routes in epithelial cells (Yoshimori, T., P. Keller, M.G. Roth, and K. Simons. 1996. J. Cell Biol. 133:247-256). To determi ne the role of Rab8 in fibroblasts, we used both transient expression systems and stable cell lines expressing mutant or wild-type (wt) Rab8 . A dramatic change in cell morphology occurred in BHK cells expressin g both the wt Rab8 and the activated form of the GTPase, the Rab8Q67L mutant. These cells formed processes as a result of a reorganization o f both their actin filaments and microtubules. Newly synthesized vesic ular stomatitis virus G glycoprotein, a basolateral marker protein in MDCK cells, was preferentially delivered into these cell outgrowths. B ased on these findings, we propose that Rab8 provides a Link between t he machinery responsible for the formation of cell protrusions and pol arized biosynthetic membrane traffic.