Structural Studies On SNX16and SNX11

Sorting nexins (SNXs), a large family proteins containing SNX-PX domain, are involved in diverse intracellular endosomal trafficking pathways. The PX domain binds to certain phosphatidylinositols (PIs) and recruites the PX domain containing proteins to the membrane rich in those PIs. Up to date,33SNXs have been found in mammalia. According to the domain composition, those SNXs can be divided into three classes:SNXPX, SNXPX-BAR and SNXPX-other.SNX16belongs to SNXpX-other, and is proposed to contain an N terminal PX domain and a C terminal Coiled coil domain. Previous studies showed that SNX16can be recruited to early endosome and late endosome, and is proposed to regulate EGFR trafficking from early endosome to late endosome.In Drosophila melanogaster, SNX16functions to regulate synaptic growth receptor and synaptic growth signaling. Researches also showed that SNX16maybe involve in some diseases, such as vesicular stomatitis virus infection, bladder cancer and hepatitis C virus replication.Here we report the crystal structure of SNX16100-278(residues100-278) at resolution of3.3A and3.35A, in P21and P212121space group, respectively. In the crystal structure, SNX16100-278, containing PX domain and Coiled coil domain, forms a dimeric structure resembling, like a pair of shears. SNX16100-278dimer contains unique PI3P binding pocket, formed by the PX domain and the dimeric coiled coil. By comparing with the crystal structure of mCISK-PX, we found that the conformation of the PPII/a2Loop mediates the size of the conventional PI binding pocket, and it may determine the PX domain binding specificity to different PIs.SNXPX is a group of proteins that contains PX domain only. It consists of SNX3-. SNX10, SNX11, SNX12, SNX22and SNX24. The structures of Grdl9p (yeast homologue of SNX3), SNX12and SNX22have been determinated. All of these structures show the conventional PX domain only. Recently some SNXPX proteins have been investigated in various biological processes. SNX3has been reported to regulate the recycling of Wntless through a novel retromer-dependent pathway. SNX10is involved in the homeostasis of endosome and is required for primary cilia formation. However, the structural basis of these activities remains largely unknown.SNX11is a SNXPX member and highly homologous with SNX10. We started to investigate the crystal structure of SNX10and SNX11, but we only obtained SNX11 crystal. We determinated two SNX11structures, SNX11-142C (residues7-142) and SNX11-170C (residues7-170), with resolution of1.78A and1.6A, respectively. The SNX11-170C structure shows that there are two a helices (a4and α5) downstream of the conventional PX structure. The two helices interact with the preceeding conventional PX structure through hydrophobic and hydrophilic interactions. a4, a5and this conventional PX structure form a stable globular domain, and we named this domain PX-extended (PXe) domain. Amino acid sequence alignment showed that SNX10may also contain this novel PXe domain.