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Updated: Jun 25, 2026

Co-immunoprecipitation Assay for Studying Functional Interactions Between Receptors and Enzymes
Published on: September 28, 2018
Unfair competition governs the interaction of pCPI-17 with myosin phosphatase (PP1-MYPT1)
Joshua J Filter1, Byron C Williams1, Masumi Eto2
1Department of Molecular Biology and Genetics, Cornell University, Ithaca, United States.
Abstract:
The small phosphoprotein pCPI-17 inhibits myosin light-chain phosphatase (MLCP). Current models postulate that during muscle relaxation, phosphatases other than MLCP dephosphorylate and inactivate pCPI-17 to restore MLCP activity. We show here that such hypotheses are insufficient to account for the observed rapidity of pCPI-17 inactivation in mammalian smooth muscles. Instead, MLCP itself is the critical enzyme for pCPI-17 dephosphorylation. We call the mutual sequestration mechanism through which pCPI-17 and MLCP interact inhibition by unfair competition: MLCP protects pCPI-17 from other phosphatases, while pCPI-17 blocks other substrates from MLCP's active site. MLCP dephosphorylates pCPI-17 at a slow rate that is, nonetheless, both sufficient and necessary to explain the speed of pCPI-17 dephosphorylation and the consequent MLCP activation during muscle relaxation.
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