The differential phosphorylation of free cytoplasmic and membrane-bound polysomal poly(A)mRNP particles in the bovine adrenal cortex

Abstract

Free and membrane-bound polysomes were isolated and incubated under in vitro conditions that allowed the phosphorylation of contained poly(A)mRNP complexes. The three phosphoproteins of 150,000, 67,000 and 45,000 daltons were consistently present in the free poly(A)mRNP fraction and absent in the membrane-bound particles. The major phosphoprotein contained in the membrane-bound poly(A)mRNP complexes was 105,000 daltons. This protein was also present in the free cytoplasmic poly(A)mRNP fraction. It is conceivable that protein kinases differentially regulate these two classes of poly(A)mRNP particles through specific phosphorylations of their associated proteins.