POLY(RIBOADENYLATE)-CONTAINING MESSENGER-RIBONUCLEOPROTEIN PARTICLES OF CHICK EMBRYONIC MUSCLES

Poly(A)-containing cytoplasmic messenger ribonucleoprotein (mRNP) particles were isolated from subcellular fractions of 12-14-day-old chick embryonic muscles by chromatography on oligo(dT)-cellulose and elution of the bound mRNP with 50% formamide. Two types of mRNP, free or nonpolysomal and polysome-derived, were obtained from the postpolysomal supernatant fraction and EDTA-dissociated polysomes, respectively. The mRNP were characterized by the absence of ribosomal RNAs and typical ribosomal proteins, by the presence of a limited number of characteristic proteins, and by the polydisperse sedimentation of the particles and their RNA moieties in sucrose gradients. Although the two types of particles show many similarities, their buoyant densities and NaDodSO4-gel electrophoreto-grams of their protein moieties indicate that they represent two distinct types of macromolecular complexes. The free mRNP are relatively protein rich and contain 10 distinct proteins in the 40000-100000 molecular weight range. The polysome-derived mRNP are comparatively protein deficient and show two major proteins of about 52 000 and 78 000 daltons, which appear to be common to both classes of mRNP. The gel patterns of the poly(A)-associated proteins in both classes of mRNP are indistinguishable, suggesting that the complex protein patterns of the free mRNP are due to the presence of a set of additional proteins which are associated with the nonpoly(A) regions of their mRNA moieties. The presence of a major protein of 78 000 molecular weight in the poly(A)-protein fragments of both classes of mRNP suggests that the association of this protein to the poly(A) tracts is not influenced by translation. The distribution of pulse-labeled total poly(A)+-RNA and two muscle-specific mRNAs, myosin heavy-chain mRNA and actin mRNA, between free mRNP and polysome fractions was about 35:65, respectively, and did not change during embryonic development between 11 and 17 days, suggesting that the two types of mRNP may exist in equilibrium in embryonic muscle cells. The possible relevance of these findings to a mechanism of translational control involving mRNP particles is discussed. © 1979, American Chemical Society. All rights reserved.