| Research Abstract |
CD45 is a critical regulator of T and B cell activation. However, there have been conflicting results as to the mode of CD45 action.
1. To obtain some insights into this problem, we tried to identify substrate PTKs for CD45 in B cells and to determine the dephosphory lation sites of the Src-family PTKs. The results demonstrated that in CD45-deficient clones from the immature B cell line, Lyn was selectively hyperphosphorylated and activated in the absence of B cell receptor (BCR) ligation. CD45 constitutively inactivates Lyn by dephosphorylating both the positive (394) and negative (508) tyrosine residues (J.Immunol. 163 : 1321-1326, 1999).
2. We also examined contribution made by CD45 to BCR-induced activation of mitogen-activated protein kinase (MAPK) family members. We found that CD45 regulated BCR-induced c-Jun NH2-terminal kinase (JNK) and p38. Interestingly, the regulation was depend on B cell differentiation level (FEBS Lett. 490 : 97-101, 2001).
3. CD45 exerts a decisive effect on selective sets of CD40-mediated signaling pathway, dictating B cell fate (J.Biol. Chem. 276 : 8550-8556, 2001).
4. To elucidate the molecular basis of these observations, we are now trying to determine the precise localization of CD45 with Src-family PTK, we utilized glycolipid-enriched membrane fraction (GEM), commonly referred to as lipid raft, as a marker. The preliminary results revealed that CD45 is present in the raft before BCR ligation and is sequestered from the raft after stimulation. These results suggest that CD45 constitutively associated with the raft inactivates, Src-family PTK and BCR ligation somehow sequesters CD45 from the raft, thereby initiating activation cascades.
Thus, we are in the process of revealing the regulatory mechanism of BCR signals by CD45 on lipid raft.
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