Prenylation of RabGTPases, Their Delivery to Membranes, and Rab Recycling - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 3

manner. While many different classes of proteins are involved in the overall
process, a crucial regulatory role is played by members of the Rab family of
small GTPases (Hutagalung and Novick 2011 ; Stenmark 2009 ). In order to fulfill
their role, the 60 members (in humans) have to be modified by addition of
geranylgeranyl groups (one, or in most cases two) to the C-terminus in a process
referred to as prenylation. These prenyl groups endow the Rab proteins with a high
affinity for membranes and allow them to play various roles, for example
connecting vesicles to systems of motor-proteins or recognizing specific tethering
factors that guide vesicles to a specific membrane or membrane compartment as a
first step in the fusion process. While the principles involved in these events are
quite well understood in a number of cases, one essential but unclarified property
that Rab proteins must have in order to play the role envisaged for them is
localization to a specific membrane or membrane domain. In this review, we will
first discuss the events leading to prenylation, then discuss the principles involved
in delivery of Rabs to membranes, and finally summarize the status of our knowl-
edge on the targeting of Rab proteins to specific membranes.
1.2 Prenylation of Rab Proteins
Rab proteins differ from Ras and Rho proteins, which are also prenylated near their
C-terminus, by not having a specific prenylation recognition sequence motif. Ras
and Rho proteins have the so-called CaaX box at their very C-termini, with the
nature of the X residue determining whether farnesylation (Ras proteins) or
geranylgeranylation (Rho proteins) occurs. In Rab proteins, the C-terminal motifs
are commonly CC, CXC, or CCXX, but in a few cases CXXX motifs occur (Rab8,
Rab13, Rab18, Rab23, Rab38). The explanation for the lack of a specific
prenylation motif is that specificity is in fact generated via an accessory protein,
the Rab escort protein (REP), which interacts with all RabGTPases (but not other
members of the Ras superfamily) and with Rab geranylgeranyl transferase
(RabGGTase), presenting the Rab C-terminus to the active site of the prenylating
enzyme. This leads to the curious situation of having a specific enzymatic modifi-
cation of a group of substrates with similar overall structure but differently struc-
tured modification sites, with the added peculiarity that the modifying enzyme has
little or no (measurable) affinity for its substrates. The latter point is put into
perspective if the REP:Rab complexes are regarded as the substrates, and these
do indeed have affinity and specificity for RabGGTase. In the presence of the
substrate geranylgeranyl pyrophosphate, i.e., the lipid donor, the affinity of
RabGGTase for REP:Rab complexes is in fact very high [ K d in the nM range
(Thoma et al. 2001a , b )].
The Rab prenylation reaction has been investigated in detail in a number of
publications, and many of the data obtained are summarized in Fig. 1.1
(Wu et al. 2009 ). The importance of the CIM (CBR interacting motif, where
CBR means C-terminus binding region) region of Rab proteins was noticed in