Novel players and large-scale protein dynamics of BCR activation revealed by APEX2 proximity labelling of lipid rafts

Abstract

Successful B cell activation, critical for high-affinity antibody production, is controlled by the B cell antigen receptor (BCR). While the main components of the BCR signalling machinery are relatively well-characterised, we still lack a comprehensive protein-level view of the very dynamic multi-branched cellular events triggered by antigen binding. In this study, we employed APEX2 proximity biotinylation to study BCR signalling-induced changes at the vicinity of the plasma membrane. APEX2 provides critical spatial and kinetic resolution, 20 nm and 1 min, to trace early protein dynamics upon antigen receptor triggering. We identified a total of 1677 proteins to locate in the proximity of APEX2 at the plasma membrane lipid raft domains, where the BCR enriches upon activation. Our data provides unprecedented insights into the composition of lipid raft environment in B cells and the dynamic changes triggered upon IgM BCR cross-linking. In addition, the data provides new insights into the behaviour of proteins known to be involved in the proximal antigen receptor signalling and simultaneously triggered processes, such as actin cytoskeleton remodelling and endocytosis. Interestingly, our differential enrichment analysis identified dynamic responses in various proteins previously not linked to early B cell activation. Furthermore, we validated Golga3 and Vti1b as novel proteins responding to BCR activation, confirming that our dataset serves as a valuable tool for future studies.