Membrane composition and curvature in SNX9-mediated actin polymerization

Area of Science:

• Biochemistry
• Cell Biology
• Molecular Medicine

Background:

• Sorting nexin 9 (SNX9) is a scaffold protein implicated in viral uptake, inflammation, and cancer progression.
• SNX9 participates in endocytosis and the formation of mitochondrial-derived vesicles.
• Its Bin-Amphiphysin-Rvs (BAR)-Phox homology (PX) domains bind phosphoinositides, and its Src homology 3 (SH3) domain interacts with proteins regulating actin polymerization.

Purpose of the Study:

• To investigate the membrane-binding specificity and actin-remodeling activities of SNX9.
• To elucidate the structural mechanisms underlying SNX9's functions at the membrane.

Main Methods:

• Biolayer interferometry to assess binding affinities.
• Cell-free reconstitution assays to study protein interactions.
• Superresolution microscopy (3D-STORM) and cryo-electron tomography to visualize SNX9 structures and actin networks.

Main Results:

• SNX9 exhibits preferential binding to liposomes containing PI(4,5)P2 and PI(3)P over PI(3,4)P2.
• Actin assembly is dependent on the combined interactions of SNX9's PX-BAR and SH3 domains.
• SNX9 forms both flat and curved assemblies at actin incorporation sites and builds branched and bundled actin networks.

Conclusions:

• SNX9 possesses multifunctional capabilities in actin remodeling.
• Its specific membrane interactions and ability to organize actin networks highlight its critical roles in cellular processes.
• Understanding SNX9's mechanisms may offer insights into cancer and viral pathogenesis.