Proteins viral-transforming

B. Locating Gene Functions - The goal of mapping a mutant is to determine what function is affected by the mutation, and where that function is located on the physical map. Consequently, the protein or protein region responsible for the function can be determined. The quest to Identify the gene function(s) required for viral transformation and how that function is controlled has been of special interest. 

Temperature-sensitive Mutants - A number of temperature-sensitive mutants of SV40 have been analyzed. Those found defective in the complementation A group and the physical map regions which code for the early proteins (T antigen and t) were also found defective in their ability to transform cells. The temperature-sensitive transformation defect was reversible at permissive temperatures, i.e., transformed colonies appeared. Mutants defective in the formation of late proteins (viral structural proteins) were able to transform cells. Therefore, functional early proteins are required to effect and maintain transformation. The early proteins necessary for transformation also permit viral DNA replication and the stimulation of host cell DNA synthesis. The exact biochemical functions required for transformation have not yet been determined. 

It can be seen then that the metabolic state of the cell is an important factor influencing surface membrane functions. Where viral transformation causes cancer-like properties, metabolic control at the nucleic acid level is likely, although viral-host interactions seem more complex than first theorized (Altman and Katz, 1976). Receptors for enteroviruses have been reported and shown to be specific for various viral strains. Susceptibility to viral infection is correlated with the presence of receptor sites on intracellular membranes as well as on the cell surface. Chemically, virus receptors solubilized from plasma membranes have been determined to be lipoproteins, with the protein moiety being most important in determining receptor activity (McLaren et al., 1968). A review of cell membrane receptors for viruses, antigens and... 

If this altered ganglioside metabolism is related to some viral function, is it a function involved in transformation, productive infection, or both Mouse cells are permissive for polyoma virus and are rarely transformed by it (cf. Eckhart, 1969). Upon viral infection, the cells undergo a series of discrete events which result in cell lysis and release of new virions (cf. Weil and Kara, 1970). These include the appearance of viral-specific T-antigen, induction of cellular and viral DNA synthesis which is maximum at 25-30 hr postinfection, synthesis of viral coat proteins and new virions (present at 50 hr.), and, finally, cell lysis and virus release (5-7 days after infection). Swiss 3T3 and TAL/N cells were treated with sufficient polyoma virus to ensure infection of all cells. Hematoside UDP-GalN AcA-acetylgalactosaminyl-transferase was then determined at two key times after infection (Table V). The activity of this enzyme was similar to that seen in mock-infected cells. The results indicate that the reduced aminosugar transferase activity in virally transformed cells is specifically related to some transforming function of the viruses and is not a consequence of lytic infection of the cell. 

Other forms of regulatory phosphorylations have been documented as well. For example, DNA transcription and replication may be regulated by phosphorylation of histones on serine, histidine, or, lysine residues (Isen-berg, 1979 Chen et al, 1974). Proteins with phosphotyrosine amino acids have often been found in the viral transformation processes (Hayman, 1981 Martensen, 1982). Also, 5 -adenylyl-0-tyrosine has been isolated as the known regulator in the adenylation of glutamine synthetase (Shapiro and Stadtman, 1968). 

NCC and NK-like cells have been reported to be involved in non-specific cell-mediated cytotoxicity (CMC) in fish. NCCs were originally described in channel catfish and are the most extensively studied killer cell population in teleosts. NCCs spontaneously kill a wide variety of target cells including tumour cells, virally transformed cells and protozoan parasites, and express components of the granule exocytosis pathway of CMC similar to mammalian cytotoxic lymphocytes (Froystad et al., 1998 Praveen et al., 2004). Catfish NCCs are small agranular lymphocytes that express a novel type III membrane protein termed the NCC receptor protein 1 (NCCRP-1) (Evans et al., 1984). NCC activity has been described in other fish species including rainbow trout, carp, damselfish and tilapia (Shen et al., 2002). 

Besides the cytokine receptors that lack intrinsic kinase activity but have associated JAK kinases, STAT proteins can be activated by a variety of G-protein coupled receptors and growth factor receptors with intrinsic tyrosine kinase activity (for example EGF, PDGF, CSF-1, and angiotensin receptor). Increasing evidence suggests a critical role for STAT family members in oncogenesis and aberrant cell proliferation. Constitutively activated STATs have been found in many transformed cell lines and a wide variety of human tumor entities. Numerous non-receptor tyrosine kinases and viral oncoproteins, such as v-Src, v-Abl, v-Sis, and v-Eyk, have been identified to induce DNA-binding activity of STAT proteins. 

Protein kinase B, or Akt, was discovered as the product of an oncogene of the acutely transforming retrovirus AKT8, causing T-cell lymphomas in mice. It encodes a fusion product of a cellular serine/threonine protein kinase and the viral structural protein Gag. This kinase is similar to both protein kinase Ce (PKCe 73% identity to the catalytic domain) and protein kinase A (PKA 68%). It differs from other protein kinases in that it contains a pleckstrin homology (PH) domain, which allows it to bind to polyphosphoinositide head groups (and also to G-protein fly subunits). To date, three subtypes have been identified a, (3, and y, all of which show a broad tissue distribution. It... 

FIGURE 8.13 Nonreceptor PTKs. These protein kinases form a large family, and most of them contain SH2 and SH3 domains. Several were originally discovered as transforming genes of a viral genome, hence names such as src or abl, derived from Rous sarcoma virus or Abelson murine leukemia virus, respectively. (Adapted from Hunter, T., Biochem. Soc. Trans., 24(2), 307-327, 1996.)... 

For example, c-Fos is heavily phosphorylated on a series of serine residues in the C-terminal domain of the protein by several types of protein kinases. The likely functional importance of these phosphorylation sites is indicated by the fact that the difference between c-Fos (the normal cellular form of the protein) and v-Fos (the viral oncogene product) is a frameshift mutation in the v-Fos protein, which obliterates the phosphorylated serine residues. It is speculated that the loss of these phosphorylation sites removes one mechanism by which the cell can regulate the protein, thereby leading to cellular transformation. 

In the proper cellular context in B cells, NK cells and mast cells, Syk kinase has been shown to transduce cell growth and survival signals by activating PI3K/Akt and Ras/mitogen-activated protein kinase pathways [3,46]. When deregulated, Syk activity can promote myelodysplasias, leukemogenesis, and perhaps viral-media ted cellular transformations [47,48]. By contrast, Syk has also been proposed to function as a tumor suppressor [49]. This is however, based on... 

The ability of IFNs (especially type I IFNs) to induce an anti-viral state is unlikely to be solely dependent upon the enzymatic mechanisms discussed above. Furthermore, the 2 -5 A synthetase and eIF-2a kinase systems may play important roles in mediating additional IFN actions. The ability of such systems to stall protein synthesis in cells may play a role in IFN-induced alterations of cellular differentiation or cell cycle progression. They may also be involved in mediating IFN-induced anti-proliferative effects on various transformed cells. 

---