Effectors of Rab GTPases: Rab Binding Specificity and Their Role in Coordination of Rab Function and Localization - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 55

binds as well to Rab22 and Rab24 (subset-2) (Table 3.3 , Eathiraj et al. ( 2005 )).
The Rab-subset key residues interacting with the effector domains, identified by
structural analysis, exhibit clear conservation within the subset, and vary between
different subsets (Eathiraj et al. 2005 ). Structure-based mutational analysis
demonstrated that Rab-subset sequence conservations can account for the selective
recognition of distinct effector domains (Eathiraj et al. 2005 ).
Other examples include RILP, a well-characterized Rab7 effector that interacts
with Rab34 and Rab36 (Matsui et al. 2012 ), and MyoVa that interacts with three
subsets of Rab proteins using three distinct RBDs (Lindsay et al. 2013 ). The first
subset, consisting of Rab6 and Rab14, binds to the central stalk of MyoVa. The
second subset (Rab8 and Rab10) interacts (dependent on exonD) in a region
localized between the globular tail domain (GTD) and the central stalk (Roland
et al. 2009 ; Lindsay et al. 2013 ). The Rab11-Rab3-Rab39b subset interacts with
the MyoV GTD (Lindsay et al. 2013 ). Of note, the interaction between the MyoVa
central stalk and Rab6/Rab14 can be greatly impaired by mutation of a single
MyoVa amino acid. The finding that this amino acid is a tyrosine suggests that
phosphorylation events could be involved in the selective recognition of Rab
GTPases binding to the same domain of an effector (Lindsay et al. 2013 ). The
interaction between the MyoVa/b GTD and the Rab11-Rab3-Rab39b subset is
abolished by point mutations of three MyoV residues (Lindsay et al. 2013 ), which
are involved in Rab11a binding (Pylypenko et al. 2013 ). However, another MyoV
Gln residue mutation, which also disrupts the interaction with Rab11a, does not
affect Rab3a and Rab39b binding (Lindsay et al. 2013 ). This may indicate that the
MyoV GTD binds to different Rab GTPases using different but overlapping binding
sites or that a different subset of residues is involved in the interaction at the same
binding site. Further structural studies are necessary to understand how MyoV can
bind phylogenetically diverse Rab proteins using the same binding site.
It is likely that many other Rab effectors that can bind several Rab GTPases via
the same domain remain to be discovered. This will help to understand the phys-
iological relevance of promiscuity observed in some Rab:effector interactions. One
hypothesis is that promiscuous effectors fulfill generic functions. For instance, the
ability of OCRL to bind different Rab proteins could reflect the significance of the
Rab and OCRL-dependent remodeling of lipid compositions for various stages of
intracellular transport (Mehta et al. 2014 ) as well as for the completion of cytoki-
nesis (Dambournet et al. 2011 ). For other Rab effectors though, the promiscuity in
in vitro experiments might not be of physiological relevance. In these cases,
specificity might simply be unimportant as other factors (such as the correct
membrane composition and other interaction partners, as discussed earlier) are
necessary to achieve correct targeting of the effector protein to a certain membrane.
The “off-target” Rab and the effector would not be present on the same membrane
in vivo and therefore there was no evolutionary necessity to develop a clear binding
specificity for a certain Rab protein.