I-Protein, a New Regulatory Protein from Vertebrate Skeletal Muscle

III. Function

Abstract

I-protein inhibited the Mg-activated ATPase [EC 3. 6. 1. 3] activity of actomyosin by approxi-mately 50% at low ionic strength. Concomitantly, the onset of superprecipitation was re-tarded. I-protein was found to bind to myosin, but not to F-actin.
The inhibitory action of I-protein occurred only in the absence of Ca ions in the troponin-tropomyosin-actin-myosin system. Addition of Ca ions abolished the effect. Thus, it is very likely that I-protein prevents unnecessary hydrolysis of ATP in the relaxed state of muscle.