Purification of messenger ribonucleoprotein particles from rabbit reticulocytes by zonal centrifugation in metrizamide

Importance of polysomal mRNA-associated polypeptides for protein synthesis initiation in yeast

The polysomal mRNA from the cell-free system of the yeast Saccharomyces cerevisiae, in the absence of exogenous energy, binds to the 40S ribosomal subunit thus forming a 48S preinitiation complex which, with energy added, is converted into 80S initiation complex. By using ribosomes with a high affinity to polysomal mRNA (pmRNA) from an edeine-resistant mutant of S. cerevisiae in place of wild-type ribosomes, increased quantities of the 48S preinitiation complex are obtained. The pmRNA is found associated with several polypeptides having molecular masses of 115-98 kDa, 72 kDa, 60 kDa and 51 kDa. These polypeptides, labelled with 125I, interact with 40S and 80S ribosomes and are essential for the formation of the 48S and 80S initiation complexes inasmuch as deproteinized pmRNA alone cannot initiate the process. In addition, other polypeptides present in the cytosol are required to carry out the above-mentioned steps of protein synthesis.

Identification and properties of the 38 000-Mr poly(A)-binding protein of non-polysomal messenger ribonucleoproteins of cryptobiotic gastrulae of Artemia salina

The Mr-38 000 poly(A)-binding protein interacts with synthetic and natural RNA. A sequence-independent stoichiometry of one protein per 8 - 12 nucleotides is measured by filter binding and sucrose gradient centrifugation. Specificity for the poly(A) sequence is demonstrated from poly(A)/RNA mixing experiments. The poly(A)-binding protein has been identified as the helix-destabilizing protein HD40[Marvil, D. K., Nowak, L. and Szer, W. (1980) J. Biol. Chem. 255, 6466 - 6472] and is characterized by the existence of at least seven ionic species with a pI ranging from 9.2 to 6.6. Acidic ionic species are generated by phosphorylation with mRNP-associated protein kinase. Different ionic species are present on free mRNP and ribosomes-mRNP preinitiation complexes. The poly(A)-binding protein affects mRNA translation and (A)4 polyadenylation. The multifunctionality of the protein is discussed.

Identification of casein kinase II and phosphorylated proteins associated with messenger ribonucleoproteins particles from reticulocytes

Messenger ribonucleoprotein (mRNP) particles, isolated from reticulocyte polysomes and purified by buoyant density centrifugation in metrizamide, contained an endogenous protein kinase activity. The cyclic-nucleotide-independent protein kinase phosphorylated casein using either ATP or GTP as the phosphoryl donor and had properties similar to casein kinase II, an enzyme previously purified and characterized from the post-ribosomal supernate of reticulocytes. Antibody prepared to casein kinase II was shown to inhibit the protein kinase activity in the mRNP particles. The endogenous enzyme phosphorylated four peptides (Mr 125 000, 107 000, 76 000 and 63 000) in the mRNP particle. Three of the four peptides, plus another (Mr 175 000), were phosphorylated by purified casein kinase II while two peptides (Mr 95 000 and Mr 76 000) were phosphorylated with casein kinase I. The mRNP particles were not substrates for the cAMP-dependent protein kinases.