Control of membrane trafficking by Rab GTPases activating proteins

• Applicant: Professor Dr. Francis Barr
• Subject Area: Cell Biology
• Term: from 2006 to 2009
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 23633737

Eukaryotic cells are divided into discrete membrane-bounded compartments connected by selective membrane trafficking and fusion pathways. A variety of different guanine-nucleotide binding proteins function in many aspects of these membrane trafficking events, in general by controlling the assembly of cytoplasmic protein complexes at membrane surfaces [1-3]. This function exploits their ability to cycle between two conformational forms specified by the bound nucleotide, GDP or GTP [4]. In the case of Rabs, the activated GTP-bound form is membrane bound, while the inactive GDP-bound form can be either cytoplasmic or membrane associated [3, 5]. Rabs are attached to membranes through a hydrophobic prenylated C-terminal region, and thus require an additional chaperone-like factor in order to be released into the aqueous cytoplasm [5]. The guanine-nucleotide dissociation inhibitory factor (GDI) fulfils this function by binding to the GDP form of the Rabs and masking the prenylated C-terminus [5, 6]. Due to the low rates of nucleotide exchange and hydrolysis additional proteins termed GDP-GTP exchange factors {GEFs) and GTP-hydrolysis activating proteins (GAPs) regulate this cycle [3, 7]. Together with GDI and a set of integral membrane factors of the Yip/Yif family that bind to prenylated proteins, GEFs are thought to control the site at which a specific Rab is activated [5, 8]. In contrast, GAPs are thought to control the lifetime of the activated state of the Rabs [9]. While many studies have focussed on the identification of effector proteins binding to specific Rabs, far less is known about the GEFs and GAPs that control them. Only two Rab specific GEFs are known, Sec2 for the yeast Rab Sec4[10], and Vps9/Rabex-5 for the Ypt51/Rab5 family [11, 12]. Many more GAPs are known due to the identification of a family of proteins in yeast, GAPs for Ypts (Gyps), sharing the TBC-domain [13-15]. A crystal structure for the Gyp1 TBC-domain has been solved, and together with other kinetic measurements suggests that these proteins have an argininefinger catalytic mechanism related to that of Ras GAP [15-17]. However, even in yeast with its limited complement of Rabs and TBC-domain proteins it has proven difficult to assign specific GAP activities to each Rab [7, 14], or to prove that GTP-hydrolysis is in fact essential for Rab function [18]. In human cells there are at least 60 Rabs (including splice variants) and over 30 TBC-domain proteins, which makes this task even more complex.

• DFG Programme: Research Grants
• International Connection: United Kingdom