Calpain 2 proteolysis of PTP1B results in enhanced PTP1B activity. (A) Schematic of full-length and truncated His-tagged PTP1B constructs. The arrow indicates the approximate site of calpain 2 cleavage. Equal amounts of purified PTP1B constructs were assayed for phosphatase activity against pNPP as described in Materials and methods. Activity of truncated PTP1B constructs is expressed relative to the full-length protein. Activity data are shown as the means ± SEM from three separate experiments. Asterisks indicate statistical significance compared with full-length PTP1B activity based on a t test (P < 0.0001). (B) Purified full-length (FL) or PTP1B truncated at amino acid 381 were incubated in the presence or absence of purified calpain 2 and assayed for phosphatase activity. The asterisk indicates statistical significance compared with untreated PTP1B activity based on a t test (P < 0.001). Error bars represent the SEM of at least three different determinations.