Fig. 2. Amino acid sequence homologies between CD20 and the Fc~ RI~ 8 chain. Relative amino acid positions and species of origin for the pep# de sequences are indicated to the left;(b) human,(m) mouse,(r) rat. Tl, e three regions are predominantly located within the first, second and third transmembrane domains. Boxed regions indicate conserved residues. the FceRI 13 chain are members of a new gene family. Indeed, the proposal that CD20 comprises part of a complex on the B-cell surface parallels the finding that the FceRI 13 chain is part ot a weil-defined complex involved in signaling in mast cells is.

CD20 phosphorylation CD20 is not phosphorylated in resting B cells but becomes heavily phosphory! ated following mitogen stimulation of B cells 9JJz, and is a dominant phosphoprotein in activated B cells~... d B-cell lines I. CD20 is hyperphosphorylated in hairy-cell leukemia cells, which also have unusually high levels of intracellular calcium ion [Ca2+] i (Ref. 16). Furthermore, CD20 is constitutively phosphorylated when expressed by transfected cells of diverse lineages, and treatment of transfected cells with phorbol esters induces the same shift in molecular weight as occurs with CD20 expressed by B-ceU lines 11. In addition to these results, in vitro kinase assays indicate that ubiquitous kinases such as protein kinase C (PKC), casein kinase II (CKII) and calcium/calmodulin-dependent protein kinase II (CaM-KII) can phosphorylate CD20 (Refs 9, 17-19). Indeed, phosphorylation of CD20 induced by phorbol esters is blocked by inhibitors of PKC (Refs 10, 12). Elevation of [Ca2+] i also activates CaM-KII, with a simultaneous increase in CD20 phosphorylation 19. Each of these kinases is likely to phosphorylate CD20 on different residues, resulting in different Cunctional consequences. For instance, PKC-mediated phosphorylation of CD20 inactivates CD20-associated Ca 2+ flux (see below), and this may be one molecular mechanism by which transmembrane Ca 2+ conductance is regulated in B lymphocytes'-1.