Inactive and active mutants of rab1b are not tightly integrated into target membranes

The rab1b GTPase is localized on the ER and cis-Golgi membranes and involve d in early exocytotic membrane traffic processes. To analyze the influence of rab1b conformation on specific membrane targeting processes three point mutants simulating permanently inactive or active rab1b proteins were const ructed and expressed in BHK cells. To increase the expression of the growth inhibiting rab1b S22N and rab1b N121I mutants we co-expressed the Mss4 pro tein and observed an elevated expression of the inactive rab1b mutants. Sur prisingly, only the rab1b wild-type protein shows the correct intracellular localization, and a tight membrane association. We conclude that the targe ting process of rab1b depends predominantly on GDP/GTP exchange.