The role of the small FTPase rab4 in endosome function

Abstract

An important goal of studying membrane transport is to identify and characterize the regulatory proteins. RabGTPases are known to play an important role in the regulation of membrane transport (Mohrmann and van der Sluijs, 1999; Olkkonen and Stenmark, 1997; Takai et al., 2001). Initially it was thought that a rabGTPase binds to the membrane of a transport vesicle and recognizes a docking protein on the acceptor membrane. Hydrolysis of GTP was then thought to catalyze membrane fusion (Bourne, 1988). Also SNARE proteins are known to be important in determining specificity in membrane transport. In the original model specific t-SNAREs on the target membrane recognize and interact with specific v-SNAREs on the transport vesicle (Rothman, 1994; Sollner et al., 1993). Nowadays it is becoming clear that the mechanisms involved in docking and fusion of transport vesicles with the acceptor membrane are not that simple as was first thought. A close interplay between rab proteins and SNARE proteins has been shown to regulate membrane fusion. Rab proteins recruit specific effector proteins to membranes, which on their turn attract other cytoplasmic proteins to form a large hetero-oligomeric complex, involved in docking and fusion (McBride et al., 1999; Simonsen et al., 1999; Tall et al., 1999; Wurmser et al., 2000). This thesis describes work focussed on rab4, that is functionally associated with early endosomes. Chapter 2, 3 and 4 deal with the characterization of the function of rab4 in early endocytic membrane transport in both interphase and mitotic cells. Chapter 5 and 6 describe the search and isolation of rab4 interacting proteins.