Proteinaceous infectious particles See prion

protein kinase (PKR) An enzyme which catalyzes the phosphorylation of a protein, usually in the presence of a cyclic nucleotide, cyclic AMP or cyclic GMP, although kinases utilizing ATP or GTP are known. There are three main classes: serine-, threonine- or tyrosine-kinase, depending upon the amino acid that is phosphorylated. Several oncogene products have protein kinase activity.

Rubin CS and Rosen OM (1975) Annu Rev Biochem 44, 831

protein synthesis Initiation of protein synthesis in eukaryotic cells usually involves the recognition by the cellular initiation factor eIF-4F of the 7-methylguanylate cap at the 5' end of cellular and many viral mRNAs. A complex then forms between the mRNA and the 40S ribosomal subunit, met-tRNA, and another initiation factor, eIF-2. With a small number of cellular RNAs and with uncapped viral RNAs (such as picornaviral RNAs) initiation occurs at an internal ribosomal entry site (IRES), and eIF-4F is not involved. To start protein synthesis the 40S ribosomal subunit scans in a 3' direction until it encounters an AUG (methionine) codon, and if the local sequence context is favorable for initiation a 60S ribosomal subunit joins the complex, eIF-2 dissociates, and synthesis of the polypeptide chain begins. The most favorable sequence at the ribosomal binding site is CcpurineCCAUGG, also known as the Kozak sequence, but various other factors such as secondary structure of the mRNA may play a role. In some viral mRNAs the first AUG is ignored for these reasons and a later AUG is used. Depending on the virus, various modifications of the cellular machinery for protein synthesis may occur during replication, which may favor virus protein synthesis over cellular, and in some cases cellular protein synthesis is shut off completely and the cell dies.

Ball LA (1998) In Virology, vol. 1 of Topley & Wilson's Microbiology and Microbial Infections, Ninth edition, edited by BWJ Mahy and L Collier. London: Arnold, p. 115 Kozak M (1992) Annu Rev Cell Biol 8, 197

proteolytic cleavage

proteolytic cleavage Enzymatic cleavage of a protein at specific site(s), e.g. the cleavage of a polyprotein to yield structural proteins.

proteolytic enzyme An enzyme that catalyzes the hydrolysis of peptide bonds.

proteosome A complex of proteolytic enzymes where peptide cleavage of intra-cellular pathogens such as viruses occurs before transport via peptide transporters (TAP1 and 2) to the endoplasmic reticu-lum where they associate with the MHC class I heavy chain. The light chain (P2 microglobulin) associates with the heavy chain and the trimolecular complex is transported to the cell surface for antigen presentation. The complex of MHC class I with a foreign (virus) peptide is recognized by CD8+ T lymphocytes which then destroy the infected cell.

Lehner PJ and Cresswell P (1996) Curr Opin Immunol 8, 59

protomers Protein units which polymerize to form a capsomere.

proto-oncogenes The cellular genes that are transduced by retroviruses and are recognized by sequence similarity to the viral oncogenes. For example v-src, the first viral oncogene to be recognized, was found to be present in normal chicken cell DNA, and it is now believed that all vertebrate cells contain highly conserved proto-oncogenes. The transduced onco-gene may become modified by mutation so that, for example, the tyrosine kinase activity of v-SRC greatly exceeds that of the cellular gene product SRC. See onco-gene.

'protovirus' hypothesis A hypothesis promulgated by Howard Temin that retroid elements, present in all cells, may acquire additional genes encoding envelope proteins and so develop into retroviruses.

provirus The viral genome integrated as DNA into the cell genome with which it replicates. The term is usually applied to retroviruses. Can be activated spontaneously or in response to certain stimuli to produce complete virus. Analogous to a prophage.

prune belly syndrome A syndrome which results in a thin-walled protruding abdomen with wrinkled skin that occurred in a human neonate in association with hydrops fetalis following maternal infection with B19 virus.

Walther J-U et al (1994) Monatschr Kinderheilkd 142, 592

0 0

Post a comment