Antagonistic regulation of F-BAR protein assemblies controls actin polymerization during podosome formation

Abstract

FBP17, an F-BAR domain protein, has emerged as a crucial factor linking the plasma membrane to WASP-mediated actin polymerization. While it is well established that FBP17 has a powerful self-polymerizing ability that promotes actin nucleation on membranes in vitro, knowledge of inhibitory factors that counteract this activity in vivo is limited. Here, we demonstrate that the assembly of FBP17 on the plasma membranes is antagonized by PSTPIP2, another F-BAR protein implicated in auto-inflammatory disorder. Knockdown of PSTPIP2 in macrophage promotes the assembly of FBP17 as well as subsequent actin nucleation at podosomes, resulting in an enhancement of matrix degradation. This phenotype is rescued by expression of PSTPIP2 in a manner dependent on its F-BAR domain. Time-lapse total internal reflection fluorescence (TIRF) microscopy observations reveal that the self-assembly of FBP17 at the podosomal membrane initiates actin polymerization, whereas the clustering of PSTPIP2 has an opposite effect. Biochemical analysis and live cell imaging show that PSTPIP2 inhibits actin polymerization by competing with FBP17 for their assembly at artificial as well as the plasma membrane. Interestingly, the assembly of FBP17 is dependent on WASP, and its dissociation by WASP inhibition strongly induces a self-organization of PSTPIP2 at podosomes. Thus, our data uncover a previously-unappreciated antagonism between different F-BAR domain assemblies which determine the threshold of actin polymerization for the formation of functional podosomes and may explain how the absence of PSTPIP2 causes auto-inflammatory disorder.