Rabbit skeletal muscle phosphorylase kinase. Interactions between subunits and influence of calmodulin on different complexes.

Abstract

Aspects of the molecular interaction and subunit structure of rabbit skeletal muscle phosphorylase kinase, (alpha beta gamma delta)4, were investigated. Exogenous addition of the delta subunit (calmodulin) stimulated the activities of nonactivated phosphorylase kinase and the alpha gamma delta complex, but not the gamma delta complex. This stimulatory effect does not seem to affect the activity-pH profile of the native kinase and is expressed at least partly through an interaction of the calmodulin with the alpha subunit, resulting in an increase in the apparent Vm parameters. Low concentrations of trifluoperazine had only slight effect on the activities of the three forms of kinase, whereas high concentrations caused nonspecific inactivation. The effects of trifluoperazine and EGTA on the activities of the alpha gamma delta and gamma delta complexes were additive; together, they inactivated the two complexes to about 10% of their original activities. The delta subunit remained tightly bound to phosphorylase kinase and the alpha gamma delta complex, even in the presence of 8 M urea, but less so with the gamma delta complex. The nonactivated kinase is more stable towards heat inactivation at 37 degrees C than the alpha gamma delta complex, whereas the gamma delta complex is least stable. Similar to the holoenzyme, limited trypsin digestion activated the alpha gamma delta complex. The pH 6.8/8.2 activity ratio of this complex increased from 0.5 to 0.9, with concomitant degradation of the alpha subunit. In contrast, the gamma delta complex is fairly inert to limited proteolysis. By using sucrose density gradient ultracentrifugation and pretreatment of the gamma delta complex with EGTA, results indicated that the gamma subunit may be a Ca2+-independent kinase, which has about 30% of the original phosphorylase kinase activity.