Fc receptors specifically bind to the Fc region of Igs to mediate the unique functions to each class of Igs. To identify a novel Fc receptor for IgM, we searched expressed sequence tag database for molecules containing Ig domains with homology to those of known Fc receptors for IgM, Fcalpha/muR and polymeric Ig receptor. As a result, we identified TOSO/Fas apoptotic inhibitory molecule 3 (FAIM3) as a possible Fc receptor for IgM. HeLa cells transfected with a TOSO/FAIM3-expression vector bound to IgM but not IgG and were able to internalize IgM-conjugated beads but not IgG-conjugated beads, suggesting that TOSO/FAIM3 is indeed a receptor for IgM (FcmuR). FcmuR protein was expressed predominantly on B-lineage cells; expression of the Fcmr transcripts was observed from the pre-B-cell stage and maintained thereafter during B-cell development. These results identify TOSO/FAIM3 as a receptor for IgM and suggest that FcmuR may serve as an uptake receptor for IgM-opsonized antigens by B cells.

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Dipeptidyl peptidase IV (DPPIV) is a serine protease, a member of the prolyl oligopeptidase (POP) family, and has been implicated in several diseases. Therefore, it seems important to develop selective inhibitors for human DPPIV (hDPPIV) that are able to control the biological function of hDPPIV. In order to elucidate the binding mode and substrate specificity, we determined the crystal structure complex of hDPPIV and diprotin A (IIe-Pro-IIe), a slowly hydrolyzed substrate of hDPPIV, at 2.2 A resolution. In this paper, we discuss the molecular interaction mechanism of diprotin A with hDPPIV based on the X-ray crystal structure.

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