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

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.