SNX9 regulates tubular invagination of the plasma membrane through interaction with actin cytoskeleton and dynamin 2

Abstract

Dynamic membrane remodeling during intracellular trafficking is controlled by the intricate interplay between lipids and proteins. BAR domains are modules that participate in endocytic processes by binding and deforming the lipid bilayer. Sorting nexin 9 (SNX9), which functions in clathrin-mediated endocytosis, contains a BAR domain, however, the properties of this domain are not well understood. Here we show that SNX9 shares many properties with other BAR domain-containing proteins, such as amphiphysin and endophilin. SNX9 is able to deform the plasma membrane, as well as liposomes, into narrow tubules and recruit N-WASP and dynamin 2 to these tubules via its SH3 domain. SNX9-induced tubulation is antagonized by N-WASP and dynamin 2 while it is enhanced by perturbation of actin dynamics. However, SNX9 also has several unique properties. The tubulating activity requires the BAR and PX domains, as well as the low-complexity (LC) domain, which binds the Arp2/3 complex. SNX9 also binds to PtdIns(4)P-5-kinases via its PX domain and its tubulating activity is regulated by phosphoinositides. In addition, the kinase activity of PtdIns(4)P-5-kinases is stimulated by interaction with SNX9, suggesting a positive feedback interaction between SNX9 and PtdIns(4)P-5-kinases. These results suggest that SNX9 functions in the coordination of membrane remodeling and fission via interactions with actin-regulating proteins, endocytic proteins and PtdIns(4,5)P 2 -metabolizing enzymes.

SNX18 shares a redundant role with SNX9 and modulates endocytic trafficking at the plasma membrane

“…SNX9 interacts with various proteins, such as dynamin, N-WASP, AP-2, Arp2/3 and PtdIns(4,5)P 2 kinases; these interactions are required for proper regulation of endocytosis (Shin et al, 2008). Vertebrate genomes express two proteins that are closely related to SNX9: SNX18 and SNX33 (Haberg et al, 2008).…”

“…7C,D) only partially rescued the endocytic defects caused by either SNX18 or SNX9 depletion, suggesting that full-length SNX9 or SNX18 is required for complete rescue. This is conceivable considering that SNX18 and SNX9 are composed of four functional domains (SH3, LC, PX and BAR) that are known to bind a variety of proteins to regulate various steps of clathrin-mediated endocytosis (Lundmark and Carlsson, 2009;Shin et al, 2008).…”

“…shRNA for SNX9 (human) was described previously (Shin et al, 2008). SNX18 shRNA (rat) was designed from nucleotides 1239-1260.…”

“…and clathrin also bind to the low-complexity (LC) region of SNX9 in a cooperative manner (Lundmark and Carlsson, 2003;Lundmark and Carlsson, 2009;Shin et al, 2008). The SNX family contains two proteins that are closely related to SNX9: SNX18 and SNX33.…”

Etiological Role of Dynamin in Charcot-Marie-Tooth Disease

“…Experimentally inhibiting the GTP hydrolysis results in overpolymerization of dynamin around elongated endocytic pit (middle). B: Electron micrograph of elongated endocytic pit decorated with dynamin (arrowheads, from Takei et al, 1995) Dynamin PRD interacts with various SH3 domain-containing endocytic proteins enriched in the synapse, including amphiphysin 1 (David et al, 1996;Takei et al, 1999;Yoshida et al, 2004), endophilin (Farsad et al, 2001), sorting nexin 9 (Ramachandran & Schmid, 2008;Shin et al, 2008), syndapin (Kessels et al, 2004), and intersectin (Yamabhai et al, 1998). Such interactions may be utilized to incorporate various functional molecules synchronously required for endocytosis.…”