| Budget Amount *help |
¥2,100,000 (Direct Cost: ¥2,100,000)
Fiscal Year 1995: ¥600,000 (Direct Cost: ¥600,000)
Fiscal Year 1994: ¥1,500,000 (Direct Cost: ¥1,500,000)
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| Research Abstract |
Dipeptidyl peptidase IV (DPPIV) is an ectoenzyme and has the active site sequence Gly-Trp-Ser-Tyr-Gly (positions 629-633) which corresponds to the consensus sequence Gly-X-Ser-X-Gly proposed for the active site of serine proteinases. DPPIV was reported to be deficient ina substrain of Fischer-344 rats. Cloning and sequencing of the DPPIV cDNA from the affected rat revealed a missence mutation leading to substitution of Gly^<633>*Arg in the active site, as compared with that of the wild-type DPPIV.Pulse-chase experiments with primary-cultured hepatocytes demonstrated that the newly synthesized mutant DPPIV was retained in the endoplasmic reticulum (ER) and rapidly degraded there, resulting in no expression of the mutant enzyme on the cell surface. Site-directed mutagenesis and transfection experiments further showed that the degradation of DPPIV is caused by any single substitution of Gly^<629>, Tyr^<632> and Gly^<633>, although Tyr^<632> can be replaced by Phe for the stable expression. Thus, it is evident that the active-site motif is essential for the expression of DPPIV on the cell surface as well as for its catalitic activity. We then examined a possible mechanism for the retention and degradation of the mutants in the ER.It was found that the newly synthesized wild-type DPPIV was transiently associated with calnexin, a chaperone in the ER,and then converted to a mature form of dimer. In contrast, the mutant form was associated with calnexin for a much longer time and converted to a larger aggregated form. Taken together, these results indicate that the mutation at the active site causes misfolding of the newly synthesized DPPIV,which is retained in the ER by calnexin and undergoes rapid degradation by a proteolytic system not yet fully characterized.
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