Purification and characterization of the major 50-kDa repressor protein from cytoplasmic mRNP of rabbit reticulocytes.

A 50-kDa protein has been purified to homogeneity from free mRNP of rabbit reticulocytes. This protein, designated as p50, is present within both free mRNP (approximately 4 mol protein/mol globin mRNA) and polyribosomal mRNP (approximately 2 mol protein/mol globin mRNA). p50 is a basic protein (pI approximately 9.5) and is characterized by a high glycine content of approximately 20%. Nitrocellulose-filter analysis has shown that p50 interacts with globin mRNA with an association constant of approximately 2.5 x 10(8) M-1 (100 mM KAc, 4 degrees C). Various RNA and polyribonucleotides have the following relative affinity for p50; poly(G) > poly(U) > globin mRNA approximately 16S rRNA > poly(A) > poly(C). p50 can be phosphorylated both in vitro and in vivo.