Cytoplasmic free mRNP particles

Poly(ADP-Ribose) Polymerase Associated with Cytoplasmic Free mRNP Particles Effect of RNase A... 

Free informosomes also have been observed in the cytoplasm of cells of insect epidermis. In contrast to embryonic informosomes, they are very heterogeneous with respect to buoyant density (Katatos, 1968). Finally, free mRNP particles similar in properties to informosomes have been described in the membrane fraction of microsomes from rat brain. These particles have a heterogeneous distribution in a sucrose density gradient and a buoyant density of about 1.40 g/cm. In contrast to nuclear complexes they are not destroyed by deoxycholate treatment (Samec et al., 1968). The nature of these particles remain unclear. 

It is noteworthy that free mRNP particles are a temporary untranslatable form of mRNA in the cytoplasm and that proteins associated with the active polysomal mRNP or the repressed mRNP are different [6, 7]. There is now some evidence that ribo-nucleoprotein particles are dynamic structures and that protein exchanges occur between the cytoplasmic mRNA-associated proteins and free proteins. Involvement of mRNA-associated proteins in the regulation of protein synthesis has been considered [7-10] and post-translational modification of these proteins as a regulatory mechanism might be considered. 

ADP-Ribosylation in Cytoplasmic Ribonucleoprotein Particles Which Contain Silent mRNA. We demonstrated that ADP-ribosyl transferase activity is associated with cytoplasmic free mRNP isolated from a variety of organs and tumor cells mouse plasmacytoma, rat liver, rat brain, Krebs n cell rat brain cultured neurons and astrocytes (25-28). Table 1 summarizes the mRNP poly(ADP-ribose) polymerase activity. On a protein basis, in contrast to the tumor cells, the activity associated with rat liver and whole rat brain free mRNP is very low. Nevertheless, it is 12 fold higher than the activity associated with the microsomal ribosomal fraction reported by Burizo et al. (19). In brain mRNP and mainly in neuronal mRNP die activity is much higher than in the rat liver mRNP on a DNA basis (not shown). 

Recently, we have demonstrated the existence of a poly(ADPR) polymerase activity associated with cytoplasmic free messenger ribonucleoprotein particles (mRNP) isolated from mouse plasmacytoma cells [4]. The enzyme does not require DNA for activity and is able to produce an ADP-ribosylation of some of the mRNP proteins. We have extended our observations to Krebs II ascites-tumor cells and to rat liver. In the present report, we will discuss some properties of this enzyme, particularly the activation by RNase A. 

It appears from our results that free mRNP possesses both the enzymatic activities of synthesis and degradation of poly(ADP-ribose). A cytoplasmic poly(ADP-ribose) glycohydrolase has also recently been described by Tanuma et al. (12). ITiis suggests a balance between the ADP-ribosylation/de-ADP-ribosylation states of free mRNP proteins. In addition, free mRNP poly(ADP-ribosyl)ated proteins seem associated with different ribonucleoprotein particles which are removed from free mRNP by a 0.5 M KCl treatment. It is noteworthy that low rnoLwt. RNA able to inhibit mRNA translation in vitro have been isolated from such ribonucleoprotein particles (13). Free mRNP proteins have also been suggested to play a role in the mechanisms and regulation of the translation of mRNA (7, 8). One could speculate a role of poly(ADP-ribosyl)ation in the stmctural changes that may occur in the free mRNP to permit the translation of the mRNA repressed in these particles. 

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