| Mammalian STE 20 like kinase 2 ( MST 2 ) , a member of the STE 20 like kinase family , has been shown in previous studies to undergo proteolytic activation by caspase 3 during cell apoptosis . ^^^ In this study , we examined the mechanism of MST 2 regulation with an emphasis on the relationship between caspase 3 cleavage and protein phosphorylation . ^^^ Both the full length MST 2 and the caspase 3 truncated form of MST 2 overexpressed in 293T cells exist in a phosphorylated state . ^^^ On the other hand , the endogenous full length MST 2 from rat thymus or from proliferating cells is mainly unphosphorylated whereas the caspase 3 truncated endogenous MST 2 from apoptotic cells is highly phosphorylated . ^^^ While both the full length MST 2 and the caspase 3 truncated form of MST 2 undergo autophosphorylation , the two forms of the phosphorylated MST 2 display marked difference in susceptibility to protein phosphatases . ^^^ |