Gaf-1b is an alternative splice variant of Gaf-1/Rip11

γ-SNAP stimulates disassembly of endosomal SNARE complexes and regulates endocytic trafficking pathways

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) that reside in the target membranes and transport vesicles assemble into specific SNARE complexes to drive membrane fusion. N-ethylmaleimide-sensitive factor (NSF) and its attachment protein, α-SNAP (encoded by NAPA), catalyze disassembly of the SNARE complexes in the secretory and endocytic pathways to recycle them for the next round of fusion events. γ-SNAP (encoded by NAPG) is a SNAP isoform, but its function in SNARE-mediated membrane trafficking remains unknown. Here, we show that γ-SNAP regulates the endosomal trafficking of epidermal growth factor (EGF) receptor (EGFR) and transferrin. Immunoprecipitation and mass spectrometry analyses revealed that γ-SNAP interacts with a limited range of SNAREs, including endosomal ones. γ-SNAP, as well as α-SNAP, mediated the disassembly of endosomal syntaxin-7-containing SNARE complexes. Overexpression and small interfering (si)RNA-mediated depletion of γ-SNAP changed the morphologies and intracellular distributions of endosomes. Moreover, the depletion partially suppressed the exit of EGFR and transferrin from EEA1-positive early endosomes to delay their degradation and uptake. Taken together, our findings suggest that γ-SNAP is a unique SNAP that functions in a limited range of organelles - including endosomes - and their trafficking pathways.

The dynamic Rab11-FIPs

The Rab11-FIPs (Rab11-family interacting proteins; also known as FIPs) constitute an evolutionarily conserved protein family that act as effector molecules for multiple Rab and Arf (ADP-ribosylation factor) GTPases. They were initially characterized by their ability to bind Rab11 subfamily members via a highly-conserved C-terminal RBD (Rab11-binding domain). Resolution of the crystal structure of Rab11 in complex with FIPs revealed that the RBD mediates homodimerization of the FIP molecules, creating two symmetrical interfaces for Rab11 binding and leading to the formation of a heterotetrameric complex between two FIP and two Rab11 molecules. The FIP proteins are encoded by five genes and alternative splicing has been reported. Based on primary structure, the FIPs were subcategorized into two classes: class I [Rip11, FIP2 and RCP (Rab-coupling protein)] and class II (FIP3 and FIP4). Recent studies have identified the FIPs as key players in the regulation of multiple distinct membrane trafficking events. In this mini-review, we summarize the Rab11-FIP field and discuss, at molecular and cellular levels, the recent findings on FIP function.

Structure and dynamics of gamma-SNAP: insight into flexibility of proteins from the SNAP family

Soluble N-ethylmaleimide-sensitive factor attachment protein gamma (gamma-SNAP) is a member of an eukaryotic protein family involved in intracellular membrane trafficking. The X-ray structure of Brachydanio rerio gamma-SNAP was determined to 2.6 A and revealed an all-helical protein comprised of an extended twisted-sheet of helical hairpins with a helical-bundle domain on its carboxy-terminal end. Structural and conformational differences between multiple observed gamma-SNAP molecules and Sec17, a SNAP family protein from yeast, are analyzed. Conformational variation in gamma-SNAP molecules is matched with great precision by the two lowest frequency normal modes of the structure. Comparison of the lowest-frequency modes from gamma-SNAP and Sec17 indicated that the structures share preferred directions of flexibility, corresponding to bending and twisting of the twisted sheet motif. We discuss possible consequences related to the flexibility of the SNAP proteins for the mechanism of the 20S complex disassembly during the SNAP receptors recycling.