Phosphorylation plays positive roles in regulating the inhibitory ability of calpastatin to calpain

The objective of this study was to determine the regulatory mechanism of phosphorylation of calpastatin on the activity of mu-calpain. Crude calpastatin extracted from the longissimus lumborum (LL) muscles of three Fat Tail Han Sheep were treated with protein kinase A (PKA) or alkaline phosphatase (AP) to increase or decrease their phosphorylation levels, respectively, and then, both phosphorylated/dephosphorylated solutions were incubated with different amounts of purified mu-calpain (25, 50, 100 and 150 units). In the AP group, the rate of mu-calpain degradation was higher than the control and PKA groups. Calpastatin with higher phosphorylation levels presented much higher inhibitory abilities. Five phosphorylation sites from two phosphopeptides were identified. Protein kinase A-catalysed calpastatin phosphorylation at Ser 649 positively regulates the inhibitory abilities of calpastatin by promoting the interaction between calpastatin and calpain. These results contribute to enrich the regulatory mechanism of mu-calpain activity during post-mortem meat tenderisation.