Small nucleoprotein complex

An active bending of the DNA induced by regulatory proteins is of particular importance when a defined arrangement of the DNA in a small volume within a larger nucleoprotein complex is required. 

In the cell, the histones are associated with DNA and nucleic acid, and the proteins are extracted as a nucleoprotein complex. Deoxyribonucleoproteins contain histones and small amounts of nonbasic protein and lipids as well. The cationic groups of the histones most likely neutralize the phosphate groups of the DNA, but there is some evidence that the chromosomes contain DNA molecules not coated by histones. 

Adenovirus contains a 35-kb dsDNA complexed with four adenovirus-encoded proteins (V, VII, m, and terminal), perhaps in a nucleosome-like arrangement (Burnett, 1997). The arrangement has been controversial, as it has been suggested that the nucleoprotein is arranged both with and without overall icosahedral symmetry. Cryo-EM data support the notion that there is perhaps a small degree of icosahedral symmetry enforced by the capsid lattice, but that there is no well-structured overall icosahedral symmetry within the core (Stewart et al, 1991). 

Studies of KH clearly indicate complexity that is only partially resolved (49). DiflFerential staining reveals small, dense, homogeneous particles within amorphous KH masses, usually associated with tono-fibrils (32, 48, 49). Amino acid analyses of supposed KH materials show at least three distinctive patterns (see Table I). The amorphous material of Tezuka and Freedberg (72) has much less proline and cystine than KH studied by Matoltsy (51). Other workers have associated histidine with KH in granular cells (48, 49, 76). Tezuka s histidine values (72) fall between those of Matoltsy (51) and Hoober (76) and conceivably represent an analysis of mixed components. UgeFs bovine material is a nucleoprotein that may be either a ribosomal product or still another KH component (71, 77). 

Transport of RNA between the nucleus and cytoplasm is an essential step in the expression of messenger RNA (mRNA), and the function of ribosomal RNA (rRNA), transfer RNA (tRNA), and small nuclear RNA (snRNA). Most nuclear RNA transcripts are covalently modified, complexed with proteins into ribo-nucleoprotein particles (RNPs), and then exported through the nuclear pore complex to the cytoplasm. After export, most RNAs remain in the cytoplasm where they function in protein synthesis. However, many snRNAs are imported back to the nucleus as mature snRNP particles, where they function in pre-mRNA splicing. Small nucleolar RNAs (snoRNAs), which function in ribosome biogenesis, are not exported to the cytoplasm, but their intranuclear targeting to the nucleolus is an area of active research. 

Lastly, the complex proteins have been gathered into a separate classification. They are composed of a protein part and an additional, nonprotein, prosthetic group. It is rather difficult to maintain the distinction between proteins which adsorb metals and carbohydrates or incorporate them in small quantities, and proteins with a definite metal or carbohydrate component. The usual division of complex proteins comprises the following (1) metalloproteins, (2) phosphopro-teins, (3) lipoproteins, (4) nucleoproteins, (5) glycoproteins, and (6) chromoproteins. 

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