Polypeptides associated with the rabbit reticulocyte poly(A)-protein complex, isolated by oligo (dT)-cellulose chromatography, were analyzed by SDS-polyacrylamide gel electrophoresis. The complex derived from EDTA released total polysomal mRNP and 20S globin mRNP by RNase treatment contained four polypeptides of molecular weight 58 000, 67 000, 71000 and 79 000. A 79 000 molecular weight polypeptide, which was also a maj or component of the reticulocyte low molecular weight cytoplasmic fraction, was shown to form tenacious associations with poly(A) in vitro.
I. INTRODUCTION
The polyadenylate [poly(A)] region of messenger RNA (mRNA) serves as a protein binding site in messenger ribonucleoprotein (mRNP) particles [ 1,2]. One of the poly(A)-associated proteins, a tightly-bound, 78 000 molecular weight polypeptide, was first identified in rabbit reticulocyte m RNP [3]. Subsequent studies revealed the association of a similar polypeptide with poly(A) segments of a number of mammalian cells [3][4][5][6][7][8][9][10][11][12][13]. M ore recently it has been demonstrated that several polypeptides may be specifically bound to nuclear [ 10] and cytoplasmic [11][12][13] poly (A). We described a poly(A)-protein complex from Ehrlich ascites cells which was composed of four polypeptides exhibiting molecular weights of 56 000, 67 000, 71000, and 81 000 daltons. Since tumor cells contain an unspecialized mRNP population and possibly a heterogeneous array of poly(A)-protein complexes, we were unable to determine whether each class of complex contained a distinct polypeptide or each polypeptide species was represented in all classes of complex. In order to resolve this problem we have characterized the polypeptides associated with poly(A) in rabbit reticulocyte mRNP, a highly specialized class of particles primarily containing the globin mRNP species [ 14], and therefore more likely than tumor cells to exhibit a homogeneous poly(A)protein population.
We report the occurrence of four polypeptides in the globin poly(A)-protein complex. These polypeptides are about the same size as those previously identified in Ehrlich ascites