The role of the small GTPase rab4 in endosome function

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.