The two major classes of antigen receptors on murine B lymphocytes, mIgM and mIgD, are both contained in a complex with two additional molecules, Ig‐alpha and Ig‐beta, which permit signal transduction. Accordingly, early biochemical events after antigen binding to either receptor are similar; biological effects, however, are different. Here, we describe three newly discovered intracellular proteins of 32, 37 and 41 kDa molecular mass, that are non‐covalently associated with mIgM, but not with mIgD. These proteins coprecipitate with mIgM in Triton X‐100 and Nonidet P‐40, but not in digitonin lysates. In addition, mIgM is to some extent associated with 29 and 31 kDa proteins that are predominantly associated with mIgD (see accompanying paper). Amino acid sequencing of p32 and p37 identified p32 as mouse prohibitin; this was corroborated by Western blot analysis with antibodies specific for rat prohibitin. p37 is a newly discovered protein. cDNA clones for both proteins were isolated and sequenced. The deduced amino acid sequence of p32 is identical to that of rat prohibitin. p37 is highly homologous to p32. Since prohibitin was identified as an inhibitor of cell proliferation, its association with mIgM, but not mIgD, could explain the different biological events elicited after engagement of each receptor.

CD26 is a T cell activation antigen known to bind adenosine deaminase and have dipeptidyl peptidase IV activity. Cross-linking of CD26 and CD3 with immobilized mAbs can deliver a costimulatory signal that contributes to T cell activation. Our earlier studies revealed that cross-linking of CD26 induces its internalization, the phosphorylation of a number of proteins involved in the signaling pathway, and subsequent T cell proliferation. Although these findings suggest the importance of internalization in the function of CD26, CD26 has only 6 aa residues in its cytoplasmic region with no known motif for endocytosis. In the present study, we have identified the mannose 6-phosphate͞insulin-like growth factor II receptor (M6P͞IGFIIR) as a binding protein for CD26 and that mannose 6-phosphate (M6P) residues in the carbohydrate moiety of CD26 are critical for this binding. Activation of peripheral blood T cells results in the mannose 6 phosphorylation of CD26. In addition, the cross-linking of CD26 with an anti-CD26 antibody induces not only capping and internalization of CD26 but also colocalization of CD26 with M6P͞IGFIIR. Finally, both internalization of CD26 and the T cell proliferative response induced by CD26-mediated costimulation were inhibited by the addition of M6P, but not by glucose 6-phosphate or mannose 1-phosphate. These results indicate that internalization of CD26 after crosslinking is mediated in part by M6P͞IGFIIR and that the interaction between mannose 6-phosphorylated CD26 and M6P͞IGFIIR may play an important role in CD26-mediated T cell costimulatory signaling.T cell activation antigen CD26 is a multifunctional, 110-kDa cell surface glycoprotein (1, 2). Although constitutively expressed in the liver, intestine, and kidney, the CD26 expression level is tightly regulated on T cells and its density is markedly enhanced after T cell activation. In the resting state, CD26 is expressed on a subset of CD4 ϩ memory T cells, and this CD4 ϩ CD26 high T cell population has been shown to respond maximally to recall antigens (1, 2).CD26 has a dipeptidyl peptidase IV (DPPIV) activity in its extracellular domain that can cleave amino-terminal dipeptides with either proline or alanine in the penultimate position (3, 4). Recently, it has been reported that an amino-terminal truncation of RANTES (regulated on activation, normal T cell expressed and secreted) by CD26͞DPPIV provides a mechanism for regulation of its activity and target cell specificity (5-7). On the other hand, CD26 interacts, presumably via its extracellular domain, with CD45, a protein tyrosine-phosphatase (8). In addition, the extracellular domain of CD26 on T cells forms a complex with adenosine deaminase, which reduces the immunosuppressive activity of local adenosine by its catalytic removal (9-12). The most striking evidence for the importance of adenosine deaminase for immune function is that a defect in adenosine deaminase activity results in severe combined immunodeficiency disease in humans (13,14).CD26 is not only highly expressed on act...

The IgM and IgD classes of antigen receptor can perform different functions on B cells. However, so far no class‐specific components communicating with the cytoplasm have been found in the two antigen receptors. We have employed a new biotinylation protocol to search for intracellular membrane Ig‐associated proteins. Here we describe two proteins of 29 and 31 kDa that are associated with membrane IgD and to some extent with membrane IgM. The membrane IgM molecule is associated specifically with three proteins of 32, 37 and 41 kDa. The purification and sequencing of the two mIgD‐associated proteins revealed that they are novel proteins which are related to each other. These proteins may be the missing link between the antigen receptor and the cytoskeleton and may contribute to functional differences between membrane IgM and membrane IgD.