Nucleoproteins are any proteins that are structurally associated with nucleic
acids, either DNA or RNA. Typical nucleoproteines
include ribosomes, nucleosomes, and viral nucleocapsid proteins.
Deoxyribonucleoproteins
A deoxyribonucleoprotein (DNP) is a complex of DNA and protein. The prototypical
examples are nucleosomes, complexes in which genomic DNA is wrapped around clusters
of eight histone proteins in eukaryotic cell nuclei to form chromatin. Protamines replace histones during spermatogenesis.
Ribonucleoproteins
A ribonucleoprotein (RNP) is a complex of RNA and protein.
The enzyme telomerase, vault ribonucleoproteins, RNase P, hnRNP and small
nuclear RNPs (snRNPs), and ribosomes are ribonucleoproteins. Ribosomes
consist of one molecule of each of 50 or more ribosomal proteins along with
three different molecules of RNA in prokaryotes or four in eukaryotes.
Some
viruses are simple ribonucleoproteins, containing only one molecule of RNA and
a number of identical protein molecules. Others are ribonucleoprotein or
deoxyribonucleoprotein complexes containing a number of different proteins, and
exceptionally more nucleic acid molecules.
Functions-
In
eukaryotic cells, DNA is associated with about an equal mass of histone
proteins in a highly condensed nucleoprotein complex called chromatin. Deoxyribonucleoproteins in this kind of complex interact
to generate a multiprotein regulatory complex in which the intervening DNA is
looped or wound. The deoxyribonucleoproteins participate in regulating DNA
replication and transcription.
The
ribonucleoproteins play a role of protection. mRNAs never occur as free RNA molecules in
the cell. They always associate with ribonucleoproteins and function as
ribonucleoprotein complexes.
In
the same way, the genomes of negative-strand RNA viruses never exist as free
RNA molecule. The ribonucleoproteins protect their genomes from RNase. Nucleoproteins are often the major antigens for viruses because they have
strain-specific and group-specific antigenic
determinants.
Structure-
Through crystallographic methods,
the specific spatial structure and biological functions of many nucleoproteins
are understood. The structures of many viral nucleoproteins
have been determined, including those of influenza, rabies, Ebola, Bunyamwera,
Schmallenberg, Hazara, Crimean-Congo hemorrhagic fever and Lassa. Important techniques for detecting the structures of
nucleoproteins include X-ray diffraction, nuclear
magnetic resonance and cryo-electron microscopy.
Classification of
nucleoprotein-
1.
Histones
– These are very basic proteins, basic because they are enriched in the amino
acids arginine and lysine to a level of about 24 mole present. arginine and
lysine at physiological pH are cationic
2. Non-histones-about
50% non-histones of chromatin have been found to be structural proteins and
include such proteins as actin, and α- and β-tubulins and myosin. They vital
ingredients of the chromosome, functioning during chromosome condensation and
in the movement of chromosomes during mitosis and meiosis. many of the
remaining 50%of the non-histones include all the enzymes and that are involved
in DNA replication, in transcription and in the regulation of transcription.
These proteins are not a highly conserved among organisms, although they must
carry out similar enzymatic activities. Apparently they are not as important as
the histones in maintaining chromosomes integrity.
