Interaction of plant protein Ser/Thr phosphatase PP7 with calmodulin

We have recently identified PP7, a novel group of plant protein Ser/Thr pho sphatases, and hypothesized that PP7 may possess a calmodulin-binding site. To test this hypothesis, we assessed the effect of calmodulin on the activ ity of recombinant Arabidopsis thaliana PP7 and directly tested interaction between PP7 and calmodulin using surface plasmon resonance. Calmodulin exe rted a moderate inhibitory effect on the phosphatase activity of PP7 with s ubmicromolar affinity. PP7 specifically interacted with immobilized calmodu lin (but not with recoverin, another EF hand Ca2+-binding protein) in a str ictly Ca2+-dependent manner with nanomolar affinity. Deletion of an insert in the catalytic domain of PP7, predicted to function as a calmodulin-bindi ng site, greatly decreased PP7 binding to calmodulin. These findings provid e the first evidence for a plant protein phosphatase directly interacting w ith calmodulin and indicate that PP7 might be regulated by Ca2+ levels in v ivo. (C) 2001 Elsevier Science.