Membrane trafficking at the ER/Golgi interface: functional implications of RhoA and Rac1

N-WASP and Arp2/3, the components of the actin nucleation/polymerization signaling pathway governed by Cdc42, are located in Golgi membranes and regulate ER/Golgi interface protein transport. In the present study, we examined whether RhoA and Rac1, like Cdc42, are also involved in this early secretory pathway. Unlike Cdc42, RhoA and Rac1 were not observed in the Golgi complex of different clonal cell lines nor were they present in isolated Golgi membranes. Expression of constitutively active or inactive mutants of RhoA or Rac1 proteins in HeLa cells did not alter either the disassembly or the assembly of the Golgi complex following the addition or withdrawal of BFA, respectively, the ER-to-Golgi VSV-G transport or the Sar1(dn)-induced ER accumulation of Golgi proteins. Moreover, unlike Cdc42-expressing cells, the 15 degrees C-induced subcellular redistribution of the KDEL receptor remained unaltered. Only cells that constitutively express the activated Cdc42 mutant (Cdc42Q61L), or that were microinjected with activated Cdc42Q61L protein, exhibited a significant change in Golgi complex morphology. Collectively, our results demonstrate that RhoA and Rac1 are not located in the Golgi complex, nor do they directly or indirectly regulate membrane trafficking at the ER/Golgi interface. This finding, in turn, confirms that Cdc42 is the only Rho GTPase to have a specific function on the Golgi complex.