Calmodulin‐dependent protein phosphorylation and calcium uptake in rat‐liver microsomes

Abstract

A 20‐kDa protein becomes phosphorylated in a process stimulated by Ca²⁺ and calmodulin in the light microsomal fraction of rat liver homogenate. The uptake of Ca²⁺ in light microsomal fraction is also calmodulin‐stimulated. The stimulation of Ca²⁺ transport is associated with the operation of a protein phosphorylation system dependent on Ca²⁺ and calmodulin. Transport is inhibited by a protein phosphatase which is stimulated by Ca²⁺ and calmodulin. It is proposed that the phosphorylation of the 20‐kDa protein, which is stimulated by Ca²⁺ and calmodulin, plays a role in the regulation of the microsomal Ca²⁺, Mg²⁺‐ATPase.