Rab GEFs and GAPs: The Enigma Variations - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 92

TBC1D20 (RabGAP)

2.7Å

Q144

Q67

R105

BeF 3

GDP

K21

Rab1

Fig. 5.3 Rab GAP mechanism and structure. TBC domain Rab GAPs utilise an arginine/gluta-
mine dual finger mechanism. The interface between Rab1 ( magenta ) and its GAP TBC1D20
( cyan ) shows the Rab1 switch II glutamine 67 residue contacting the GAP backbone. Glutamine
144 and arginine 105 finger residues contributed by the GAP are in close proximity to the gamma-
phosphate position of the bound GTP, mimicked by GDP:berylium fluoride in this structure

5.3.2 Defining Cellular Functions for Rab GAPs

Although it has been relatively simple to identify the Rab GAP complement due to
the presence of the conserved TBC1 domain in these proteins, elucidating specific
cellular functions for these proteins has proven more difficult. In yeast the Gyps are
not essential for growth under standard laboratory conditions, and biochemical
analysis has provided conflicting reports on specificity. Here I will describe the
functions of a subset of Rab-RabGAP pairs in secretion and cell polarisation, and
endocytic trafficking and autophagy.
TBC1D20 and its yeast orthologue Gyp8 are ER-localised RabGAPs For Rab1
and Ypt1, respectively (De Antoni et al. 2002 ; Haas et al. 2007 ). Human TBC1D20
shows equivalent biochemical activity towards both Rab1 and Rab2, and some
weak activity towards Rab18 (Haas et al. 2007 ). Cell biological studies have
implicated both Rab1 and Rab2 in ER to Golgi trafficking (Tisdale et al. 1992 ;
Schwaninger et al. 1992 ), while Rab18 plays a role in lipid droplet formation from
the ER (Ozeki et al. 2005 ; Martin et al. 2005 ). Both overexpression and genome-
wide siRNA screening support a physiologically relevant role for TBC1D20 in the
regulation of secretion (Haas et al. 2007 ; Wendler et al. 2010 ), but have not
definitively confirmed whether Rab1, 2 or 18 or a combination of these GTPases
is the crucial cellular target. Loss-of-function mutations in TBC1D20 are found in
the blind sterile mouse, and cause the human neurodegenerative and developmental