Informosome-forming protein

Free Informosome-Forming Protein in the Cytoplasm and the Problem of the Preexistence of I nformosomes. 

Two questions arise. First, is the informosome-forming protein (IFF) also found in cytoplasmic informosomes and nuclear D-RNPs Second, are cytoplasmic informosomes preexisting structures, or are they produced artificially by the interaction of IFF with mRNA during the preparation of the cellular extract The answer to the first question has not yet been obtained, since the isolation of pure IFF meets with a number of difficulties. The first results obtained by Baltimore and Huang (1970) indicate the possibility of separating active material into a number of fractions by chromatography on DEAE-cellulose. 

One can suggest that the cell contains a pool of free informatin, synthesized in the cytoplasm, which moves to the nucleus and combines there with the chromosomal sites involved in the synthesis of D-RNA. It corresponds to informosome-forming protein of the cytoplasm. 

In 1964 Spirin et al. discovered in embryonic cells cytoplasmic ribonucleoprotein particles containing rapidly labeled RNA with many properties of mRNA. The particles contain RNA and protein in a ratio of about 1 3 or 1 4. These particles, which are free cytoplasmic mRNA-protein complexes not combined with ribosomes, were named informosomes. Under ultracentrifugation they form a number of discrete peaks with sedimentation coefficients equal to 20, 30, 40, 50, 55, 65, and 75S (Spirin et al., 1964). 

The presence of informosomelike RNPs has been demonstrated also in the case of Ehrlich ascites carcinoma cells infected with Sendai virus (Volkova et al., 1969). The cytoplasmic extracts of the cells labeled for 30 minutes with uridine contain virus-specific RNA in the form of particles with a sedimentation coefficient 45S (the sedimentation coefficient of the complete virus equals 57S). The buoyant density of the RNP peak in CsCI equals 1.43 to 1.44 g/cm. Although these properties are compatible with the idea that they are informosome, the particles, and in particular their protein component, should be characterized in more detail before reaching a definite conclusion. Recently SOS virus RNA-containing particles with p = 1.40 g/cm have been found in HeLa cells infected with poliovirus (Huang and Baltimore, 1970), although the authors have some doubts about the reality of these complexes. 

In attempts to check the in-vivo existence of informosomes, Girard and Baltimore (1966) mixed different RNAs with the cytoplasmic supernatant and observed the interaction of RNAs with the cytoplasmic proteins. The sedimentation coefficient of the RNA increased 1.5 to 2.5 times. These data were confirmed by (Ovchinnikov et al. (1968) and Spirin (1969) with cytoplasmic fractions from rat liver and loach embryo. Thus in the cytoplasm of animal cells a special protein able to form complexes with... 

Many other schemes may also be drawn. To determine which scheme is correct, it will be necessary to answer the following questions l.What is the exact localization of free cytoplasmic informosomes in the cell 2. Do nuclear D-RNP, cytoplasmic informo-somes, and polysome-bound mRNP have the same nature In particular do they contain the same protein moiety or not 3. What is the nature of intermediate complexes in the process of polysome formation and where are the polysomes formed ... 

The next step is the combining of the informosomes with ribosomes or their subunits (probably with newly formed ribosomes) which takes place immediately or very soon after their entry into the cytoplasm. This interaction is accompanied by the loss of one third to one half of the protein from the informosomes. Another half of the protein is a constant constituent of polysomes and may be engaged in the process of translation. 

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