Negative regulator of B-cell activation. Down-regulates cell proliferation (in vitro). Promotes RAC1-dependent membrane ruffle formation and reorganization of the actin cytoskeleton. Regulates cell spreading and cell polarization. Stimulates HDAC1 activity. Regulates LYN activity by modulating its tyrosine phosphorylation. Interacts with FASLG (By similarity). Interacts with phosphotyrosine containing proteins. Interacts (via SH3 domain) with CTTN. Interacts (phosphorylated at Ser-23) with YWHAB, YWHAE, YWHAG, YWHAH, YWHAZ and SFN. Interacts directly with SAP30 and HDAC1. Identified in a complex with SAP30 and HDAC1. Shuttles between cytoplasm and nucleus. Colocalizes with the actin cytoskeleton and actin-rich membrane ruffles. Detected in spleen and lymph node (at protein level). Up-regulated in peripheral blood B-cells by IL4 and bacterial lipopolysaccharide (LPS). No visible phenotype. Mice have normal bone marrow B-cell development and normal splenic T- and B-cell populations, but show an enhanced immune response upon immunization. Mice have constitutively activated Lyn, due to constitutive Lyn tyrosine phosphorylation.