Differential repression of specific mRNA in erythroblast cytoplasm: a possible role for free mRNP proteins.

Abstract

Two types of in vivo untranslated ‘free’ mRNA‐protein particles (mRNP) were isolated from duck erythroblast cytoplasm and characterised. Both types, namely the highly purified globin mRNA‐specific ‘20S’ mRNP and the ‘35S’ mRNP containing a heterogenous non‐globin mRNA population, are not translatable in rabbit reticulocyte lysates, but yield active mRNA upon deproteinisation. In vivo, 90% of globin mRNA is translated, but the majority of mRNA types are found in the inactive mRNP fraction, including fully repressed mRNA species. Searching for the factors controlling differential mRNA repression, we characterised and compared the protein composition of globin and ‘35S’ mRNP using two dimensional gel electrophoresis, in vivo labelling with [35S]methionine and in vivo phosphorylation. The major proteins ubiquitously bound to globin or any other mRNA in the polyribosomes (e.g., the 73 K mol. wt. poly(A) binding protein) were not detected in purified inactive mRNP. In the latter some polypeptides appear to be associated with only one of the two inactive mRNA types while some others are common to both mRNPs. Furthermore, different rates of synthesis and phosphorylation characterize the protein populations of the two types of repressed mRNP. The specificity in composition and metabolism of the populations of polypeptides associated with different subpopulations of inactive cytoplasmic mRNA, as shown here, argues in favour of a role of mRNP proteins in mRNA recognition and selective translational repression, possibly in association with the ScRNA previously found as components of the free mRNP and able to inhibit protein synthesis.

Identification and characterization of the poly(A)-binding proteins from the sea urchin: a quantitative analysis.

“…However, since this material was not released from polysomes by EDTA, it cannot be associated with polysomal poly(A)+ mRNA. This is a critical observation because, on the basis of gradient experiments, it has previously been reported that PABP associates only with polysomal poly(A)+ mRNA (54)(55)(56). Furthermore, EDTA controls were not performed in these studies, and polysomal PABP was isolated from a polysomal pellet.…”

“…Although mRNP proteins are hypothesized to perform roles in translational regulation, the function of most of them remain unknown (18,31,32,37,43,(54)(55)(56). To date, the four cap-binding proteins (eIF-4E and the three proteins of the eIF-4F complex) and the poly(A)-binding proteins (PABPs) are the only mRNP proteins that have been identified across species boundaries.…”

“…The ratio of PABP to adenosine residues in peak 1 was calculated to be 1:86 in eggs and 1:56 in two-cell embryos. In other systems, it has been reported that only translationally active, i.e., polysomal, poly(A)+ mRNA binds PABP (54)(55)(56). In sea urchin eggs and two-cell embryos, 50 to 65% of the RNA containing poly(A) tracts is untranslatable interspersed repeat-containing RNA (see reference 12 for a discussion).…”

Identification and Initial Characterization of a Specific Proteasome (Prosome) Associated RNase Activity

“…Several years ago, it was shown that free mRNA bound to transacting proteins (free mRNPs) are blocked for translation, while deproteinized mRNA stimulated protein synthesis in cell free systems (Schmid et al, 1983a(Schmid et al, , 1983bImaizumi-Scherrer et al, 1982;Vincent et al, 1983). Several reports support the idea that there is a close relationship between proteasomes and untranslatable free mRNPs (Schmid et al, 1984;Martins de Sa et al, 1986).…”

Light-stimulated accumulation of the peroxisomal enzymes hydroxypyruvate reductase and serine:glyoxylate aminotransferase and their translatable mRNAs in cotyledons of cucumber seedlings

“…Translational control could involve specific inhibitors which prevent the translation of preexisting mRNAs [38]. Alternatively, the synthesis of HPR and SGAT could depend on light-stimulated increases in translatable mRNA for these proteins.…”