The β12-β13 loop of protein phosphatase-1 is involved in activity regulation

Protein phosphatase-1 (PP1) is a member of the eukaryotic serine/threonine phosphatase gene family. The beta12-beta13 loop is a prominent non-conserved region among the family, and extends from the surface and overhangs the active site. To investigate the function of the beta12-beta13 loop of PP1, we systematically examined all residues by site-directed deletion mutation. Deleting residues Y272, E275 or F276, caused enzyme activity to increase, while deleting residue C273, caused enzyme activity to decrease, when G274 was deleted no remarkable activity increase was observed, and almost all activity was lost when D277, N278 or A279 were deleted. These observations implied that each amino acid has a different effect on the activity of phosphatase, which may result from their different side chains and locations. The activity change of these PP1 mutants, from Y272 to A279, was comparable to that of calcineurin mutants, from Y311 to K318. By comparison, except for D277 (N316) and A279 (K318) of PP1 (calcineurin), each pair of equivalent mutants in the beta12-beta13 loop of PP1 and calcineurin have coincident activity change although they are non-conserved, which suggests that the beta12-beta13 loop of PP1 is not only involved in activity regulation but also involved in regulation similar to that of calcineurin.