B-lymphocyte-specific membrane protein that plays a role in the regulation of cellular calcium influx necessary for the development, differentiation, and activation of B-lymphocytes. Functions as a store-operated calcium (SOC) channel component promoting calcium influx after activation by the B-cell receptor/BCR. Forms homotetramers. Interacts with the heavy and light chains of cell surface IgM, the antigen-binding components of the BCR. Constitutively associated with membrane rafts. Expressed in PBMCs and lymph node from healthy dogs, in B-cells of canine lymphoma, but not in T-cell lymphoma cells and non-T and non-B-cell lymphoma cells. Phosphorylated. Might be functionally regulated by protein kinase(s) (By similarity). Belongs to the MS4A family.