Abstract
Recently, we have found two major physiological forms of retinoid X receptor α (RXRα): the mature 54 kDa RXRα and the truncated 44 kDa RXRα lacking a portion of N-terminal A/B domain in human and rodent livers. In this communication, we show that m-calpain was active to digest 54 kDa RXRα in the human hepatoma-derived cell line, HuH7, nuclei to 44 kDa fragment through 47 kDa intermediatein vitro.Although both proteolytic fragments were revealed by anti-RXRα antibody against its E-domain, neither fragment reacted with anti-RXRα antibody specific for A/B domain. The profile of the calpain-induced proteolytic fragmentation of RXRα was almost identical to that of endogenous RXRα in nonmalignant human and normal mouse liver nuclei. This is the first demonstration that RXRα is a substrate for m-calpain, strongly suggesting that the enzyme might also be involved in post-translational modification of the receptor in hepatocytes.
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