Nuclear informosomes

The term nuclear informosomes should be used with caution. 8ince the informosomes described by 8pirin in embryonic cytoplasm seem to be well-characterized cell structures, one should compare nuclear D-RNP and informosomes before extending this term to the nuclear structures. 8ince even a comparison of the proteins of these two structures has not yet been made, for the time being we prefer to call the nuclear particles nuclear D-RNP. ... 

Thus three types of nucleoproteins containing D-RNA have been described up to now nuclear particles, informosomes (free cytoplasmic particles), and polysome-bound mRNA-proteins. Only the nuclear particles have been obtained in a purified state, and this has made it possible to study their protein composition and structural organization more extensively. Unfortunately, free informosomes and polysome-bound particles have not been characterized to a similar extent due to the difficulties in their isolation. This does not allow one to compare directly these three types of particles and to establish whether they represent the same structural elements or not. To answer this question it will be necessary to characterize the proteins constituting the cytoplasmic particles. 

The RNA of informosomes is sensitive to RNase, although somewhat more resistant than free D-RNA (8pirin, 1969). It is not clear whether mild RNase treatment transforms large informosomes to some definite minimal subunits as in the case of nuclear D-RNP. 

Many properties of informosomes can be studied only after isolation of the particles in a pure state. As we can see, free informosomes of the embryonic cytoplasm resemble in some respects nuclear D-RNPs. They are relatively homogeneous with respect to buoyant density, at least in the case of large informosomes there is a good correlation between the sizes of the informosomes and the RNAs isolated from them. They form a number of discrete components in the sucrose density gradient. All the above-mentioned properties may be explained in terms of a polysomelike structure. However, for such a conclusion it is necessary to have data about the presence of minimal units in informosomes, which data are unavailable at the moment. On the other hand, there are some differences between the nuclear D-RNPs and informosomes with respect to their sedimentation coefficients. The question of the relation between the two types of particles is discussed in detail in the section on The Problem of Messenger RNA Transport (p. 96). 

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. 

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. 

In conclusion it should be pointed out that free D-RNPs or informosomes have been found in the cytoplasm of embryonic and adult cells, but the possibility of nuclear leakage at least in the case of the cells from adult organisms has not yet been excluded. 

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 comparison of nuclear and cytoplasmic D-RNPs. In the above sections we described the properties of different types of cellular D-RNP. If nuclear D-RNA-containing particles and free informosomes are compared one can find a number of similarities. Both kinds of particles give several discrete peaks in a sucrose density gradient, which, at least in the case of nuclear D-RNPs reflects the polysomelike structure of the complexes. 

Both of them have the same buoyant density of CsCl density gradient ultracentrifugation, i.e., 1.40 g/cm. Although the informosomes from embryonic tissues are more heterogeneous and contain more than one component, the main component has this buoyant density. Informosomes containing virus-specific RNA synthesized on DNA of vaccinia virus are very homogeneous and have a buoyant density of 1.40 g/cm. Thus, the nuclear D-RNPs and free informosomes contain the same amount of protein per unit or RNA. 

Free informosomes of the cytoplasm are complexes of D-RNA with informatin, but in contrast to nuclear D-RNP they are in more extended form. 

Nonribosomal RNA appears in the cytoplasm in the form of ribonu-cleoprotein particles called informosomes (Spirin et ah, 1964 Spirin, 1969). Such ribonucleoprotein particles, which are mostly not bound to ribosomal subunits, have been found in a variety of tissues. They are in part incorporated into polyribosomes (see Lebleu et ah, 1971 Knochel and Tiedemann, 1972 Bonanou-Tzedaki et ah, 1972). The appearance of messenger-ribonucleoprotein complexes in the polysomes is a slow process as compared to the synthesis of nuclear RNA. In chick embryos newly synthesized messenger (nonribosomal) RNA is detectable in polyribosomes after about 30 minutes. The time sequence of incorporation of mRNA into polyribosomes does not change when the RNA synthesis is completely blocked after 15 minutes by actinomycin D (Knochel, 1972b). In HeLa cells mRNA appears in the polysomes after a 15-20 minutes lag phase (Penman et ah, 1968). 

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