Characteristic properties of endoplasmic reticulum membrane m-calpain, calpastatin and lumen m-calpain: a comparative study between membrane and lumen m-calpains.

Previously, we reported that bovine pulmonary smooth muscle endoplasmic reticulum (ER) membrane possesses associated m-calpain and calpastatin and ER lumen contains only m-calpain. Herein, we report characteristic properties of ER membrane m-calpain (MCp), calpastatins and lumen m-calpain (LCp) and a brief comparative study between MCp and LCp. MCp containing 80 kDa large and 28 kDa small subunit is non-phosphorylated, whereas LCp containing only 80 kDa large subunit is phosphorylated. Optimum pH, Ca(2+) concentration and pI value of both MCp and LCp are 7.5, 5 mM and 4.5, respectively. MCp and LCp have similar kinetic parameters and circular dichroism (CD) spectra. Autolysis of MCp and LCp are different. Coimmunoprecipitation studies revealed that LCp is associated with ERp57 in the ER lumen, which suggests that the regulation of LCp differs from the regulation of MCp. In presence of Ca(2+), the activated LCp cleaves inositol 1,4,5-trisphosphate receptor-1 (IP(3)R1) in the ER lumen, whereas the activated MCp cleaves Na(+)/Ca(2+) exchanger-1 (NCX1) in the ER membrane. We have determined pI (4.6 and 4.7, respectively) and IC(50) (0.52 and 0.8 nM, respectively) values of 110 and 70 kDa calpastatins. For first time, we have determined the characteristic properties, regulation and functional activity of LCp in the ER lumen.

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