Messenger Ribonucleoproteins of Higher Eukaryotes

The presence of complexes between mRNA and proteins, i.e. messenger ribonucleoprotein particles, was first discovered in the cytoplasm of animal embryonic cells (Spirin et al., 1964; Spirin & Nemer, 1965). They were called informosomes. Soon after it became clear that all mRNA in the eukaryotic cytoplasm of all cell types, at least in animals and higher plants, exists in the form of messenger ribonucleoproteins, or mRNPs.

Now several classes of mRNA-protein complexes in the cytoplasm may be distinguished: (1) Polyribosomal mRNPs, i.e. the mRNA-protein complexes within translating polyribosomes. (2) Free mRNP particles which are principally translatable, but either are in transit to polyribosomes, or represent a pool of excess mRNA for translation, or are not capable of efficiently competing with other, stronger mRNAs for initiation factors ("weak" mRNAs). (3) Non-translatable mRNP particles where initiation of translation is blocked by specific 5'-UTR-bound repressors (see Section 17.5). (4) Masked mRNP particles which are inactive in translation, stable, and stored in the cytoplasm until receiving a signal for unmasking (Section 17.6); they are typical of germ cells and other dormant states.

All the cytoplasmic mRNPs mentioned above have characteristic features in common. First, they always have a relatively high proportion of protein: the protein to RNA ratio is universally about 3:1 to 4:1 in the free mRNPs and somewhat lower, down to 2:1 in the polyribosomal mRNPs. For comparison, ribosomes have the protein to RNA ratio from 1:2 in prokaryotic particles to 1:1 in eukaryotic ribosomes. Second, at least two major families of proteins are present in stoichiometry over one protein per RNA. One is represented by a basic protein (or a couple of closely related proteins) with a molecular mass of about 35 kDa, which is usually designated as "p50", or "Y-box protein(s)" (see Section 17.2.2); this protein (or proteins) possesses a high affinity to various heterologous mRNA sequences, and much lower affinity to poly(A) tails. The other is a protein with the molecular mass of about 70 to 80 kDa (p70, or PABP, poly(A)-binding protein) having a predominant affinity to poly(A) sequences. A great variety of minor protein species are also bound within the mRNP particles. Third, the mRNP particles are found to be rather resistant to removal of Mg++, in contrast to ribosomal particles.

The protein(s) designated as p50, or Y-box protein(s) seems to be major mRNP protein component of all cytoplasmic mRNPs, both in dormant germ cells and in actively translating somatic cells. The same major mRNA-binding protein(s) can be detected both in free mRNP particles and in polyribosomal mRNPs. It seems likely that the p50 mentioned is the main protein component (mRNP core protein) physically forming the cytoplasmic mRNPs of eukaryotic cells, like histones form DNP. The role of the protein may be some kind of structural organization and sequence-nonspecific packaging of eukaryotic mRNA into mRNP particles. This universal form of the existence of eukaryotic mRNA is available for intracellular transport, translation, masking, degradation, etc., depending on other protein components involved. Under certain circumstances, with participation of a specific masking protein (see Section 17.6), the protein may be responsible for some conformational rearrangements of mRNPs, for example, their condensation into inactive (masked) particles.

Among minor protein components of mRNPs, an important role belongs to protein kinases that may govern the composition and the activity of mRNPs by inducible phosphorylation of other mRNP proteins. Also other enzymatic activities and proteins serving translation, including some initiation factors, can be found associated with mRNPs. Schematic representation of the distribution of mRNA-binding proteins among different functional regions of eukaryotic mRNA is given in Fig. 2.6.

Generally, the massive loading of eukaryotic mRNA with proteins suggests that the following points may be very important in considering mRNA interactions with the translation machinery. (1) The binding of proteins may modify, melt, induce, or switch structural elements in mRNA, thus affecting its elF's eEF's, ARSases RIBOSOMES


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