Thermolabile protein kinase molecules in a temperature-sensitive murine sarcoma virus pseudotype

Cellular kinases incorporated into HIV-1 particles: passive or active passengers?

Phosphorylation is one of the major mechanisms by which the activities of protein factors can be regulated. Such regulation impacts multiple key-functions of mammalian cells, including signal transduction, nucleo-cytoplasmic shuttling, macromolecular complexes assembly, DNA binding and regulation of enzymatic activities to name a few. To ensure their capacities to replicate and propagate efficiently in their hosts, viruses may rely on the phosphorylation of viral proteins to assist diverse steps of their life cycle. It has been known for several decades that particles from diverse virus families contain some protein kinase activity. While large DNA viruses generally encode for viral kinases, RNA viruses and more precisely retroviruses have acquired the capacity to hijack the signaling machinery of the host cell and to embark cellular kinases when budding. Such property was demonstrated for HIV-1 more than a decade ago. This review summarizes the knowledge acquired in the field of HIV-1-associated kinases and discusses their possible function in the retroviral life cycle.

Characterization of leukemogenic virus produced by a new line of Friend erythroleukemia virus-transformed cells

A new cell line designated SQ-A was established from the spleen of a leukemic DBA/2J mouse inoculated with the anemic strain of Friend erythroleukemia virus (FLV-A). The cells are similar in morphology, growth pattern, and tumorigenicity to our prototype erythroleukemia line 5-86 but are more sensitive to the cytotoxic effects of inducers of differentiation. The virus produced by SQ-A cells induces erythroleukemia associated with anemia in adult mice but has little activity when assayed on XC cells. It was characterized to determine what factors influence its leukemogenic potential. As compared to the attenuated virus from cultures of 5-86 and G-2 cells, the subunits of the RNA from the virions of SQ-A cells are the same size, and the amount of reverse transcriptase activity and RNase H present in the purified virions of the three lines are similar. However, differences are observed in levels of endonuclease and protein kinase. Both enzymes are increased in SQ-A virions. The activity of protein kinase in SQ-A virions is about 5 times higher than that in the attenuated virions. The number of polypeptides and their phosphorylation patterns also distinguish the virions of SQ-A. Whereas 5-86 virions contain seven proteins, three of which are phosphorylated in vitro, SQ-A virions contain eight proteins, all of which are phosphorylated. The extra protein in SQ-A virions has a molecular weight of 25,000 and is not glycosylated.

Action of temperature-sensitive mutants of myeloproliferative sarcoma virus suggests that fibroblast-transforming and hematopoietic transforming viral properties are related

The myeloproliferative sarcoma virus is molecularly related to the Moloney sarcoma virus (Pragnell et al., J. Virol. 38:952-957, 1981), but causes both fibroblast transformation in vitro and leukemic changes--including spleen focus formation--in adult mice. The fibroblast transforming properties of myeloproliferative sarcoma virus were used to select viral temperature-sensitive mutants at 39.5 degrees C, the nonpermissive temperature. These mutants are temperature sensitive in the maintenance of the transformed state. This was also shown by cytoskeletal changes of the infected cells at permissive and nonpermissive temperatures. Viruses released from cells maintained at both the permissive and nonpermissive temperature are temperature sensitive in fibroblast transformation functions. All temperature-sensitive mutants show only a low reversion rate to wild-type transforming function. The myeloproliferative sarcoma virus temperature-sensitive mutants are inefficient in causing leukemic transformation (spleen enlargement, focus formation) in mice at the normal temperature. A method to maintain a low body temperature (33 to 34 degrees C) in mice is described. One temperature-sensitive mutant was checked at low body temperature and did not induce leukemia. These data thus indicate that the same or related viral functions are responsible for hematopoietic and fibroblast transformation.

P85gag-mos encoded by ts110 Moloney murine sarcoma virus has an associated protein kinase activity

A protein identified as P85(gag-mos) was shown to be phosphorylated when immunoprecipitates from ts110 Moloney murine sarcoma virus transformed nonproducer cells (clone 6m2) were incubated with [gamma-(32)P]ATP. The in vitro-labeled 85,000-dalton phosphoprotein comigrated on NaDodSO(4)/polyacrylamide gels with authentic phosphorylated P85(gag-mos). Immunoprecipitates obtained with antisera prepared against Rauscher murine leukemia virus core protein p30 were active in the immune complex kinase assay but anti-murine leukemia virus p10 precipitates were not. Previous studies have shown that anti-p30 but not anti-p10 antisera recognize P85(gag-mos). The 6m2 clone has been shown to express P85(gag-mos) at 33 degrees C but not at 39 degrees C. Anti-p30 immune complexes from 6m2 cells maintained at 39 degrees C failed to phosphorylate the 85,000-dalton protein. Furthermore, the in vitro phosphorylated 85,000-dalton protein gave the same pattern of V8 protease-generated cleavage products as in vivo(32)P-labeled P85(gag-mos). We conclude from these results that P85(gag-mos) is phosphorylated in anti-p30 immune complex kinase reactions. Phosphoamino acid analyses indicated that the in vitro phosphorylated P85(gag-mos) contained phosphoserine and phosphothreonine. Our findings indicate that incubation of anti-p30 immunoprecipitates at 39 degrees C drastically reduced, in a specific way, the kinase activity associated with P85(gag-mos). This result and other data suggest that the kinase is virus-encoded. Because P85(gag-mos), but not Pr65(gag) is phosphorylated in anti-p30 immunoprecipitates from MuLV-MuSV ts110 producer cells, the kinase enzyme is associated with P85(gag-mos) and not gag gene products. A second major polypeptide of the size of P58(gag) was also phosphorylated in anti-p30 immunoprecipitates from cells maintained at 33 degrees C but not at 39 degrees C. Since 6m2 cells at 39 degrees C contain P58(gag), this is also consistent with the kinase activity being associated with P85(gag-mos).

Properties of a sarcoma-growth-factor-like peptide from cells transformed by a temperature-sensitive sarcoma virus

Serum-free conditioned media was collected from three sarcoma virus-transformed cell lines and an untransformed cell line. All three virally transformed lines produced and released growth factors into their serum-free media. The major activity in all cases, whether the cells were transformed by Moloney sarcoma virus (MSV) or Kirsten sarcoma virus (KiSV), or whether they were mouse or rat, was a sarcoma-growth-factor (SGF)-like activity with an apparent molecular weight of 10,000. The SGF-like pools from a Moloney sarcoma virus-transformed mouse 3T3 cell and a Kirsten sarcoma virus-transformed NRK cell were further purified by carboxymethyl cellulose chromatography. The elution profiles of these peptides were very similar. The serum-free conditioned media from the untransformed cells showed no detectable growth stimulating activity. The temperature sensitivity of an SGF-like growth factor from the supernate of a NRK cell transformed by a wild-type Kirsten sarcoma virus (KiSV) was compared with that of the SGF-like activity from the supernates of a NRK cell transformed by a ts-mutant of KiSV that is temperature sensitive with respect to transformation (ts-371 Cl 5). Neither the cells transformed by the wild-type sarcoma virus nor those transformed by the temperature sensitive virus released a SGF-like activity that was temperature sensitive under the conditions of the assays.

Sequence of events in the transformation process in cells infected with a temperature-sensitive transformation mutant of Moloney murine sarcoma virus

Normal rat kidney cells infected with the temperature-sensitive transformation mutant of Moloney murine sarcoma virus were used to study the biochemical and morphological changes that occur during transformation. The infected cells exhibited a normal morphology at the nonpermissive temperature (39 degrees C) and a transformed morphology at the permissive temperature (33 degrees C). A new viral protein was detected 2 hr after shift to the permissive temperature as a polyprotein with an estimated Mr of 85,000 (p85). Scanning electron microscopy of the cells within 5 hr after shifting them to the permissive temperature showed that they became smaller and rounded with numerous elongated microvilli. In an earlier study, changes in hexose uptake were found to occur 8-12 hr after the shift [Horn, J. P., Wood, T. G., Blair, D. G. & Arlinghaus, R. B. (1980) Virology 105, 516-525]. By 48 hr, the cells had the morphology of a fully transformed cell. Concomitant with the changes in the morphology were alterations in the cytoplasmic microtubule complex. At the nonpermissive temperature, the complex consisted of a lacy network of microtubules. Within 5 hr at the permissive temperature, the lacy network was still present but the microtubules were more diffusely stained and less discernible. By 48 hr, the microtubules were so diffuse that the lacy network could not be recognized. Alterations in the F-actin cables did not occur until 24 hr after shifting the cells to the permissive temperature. Enucleation of the cells at the nonpermissive temperature and shifting the cytoplasts to the permissive temperature did not result in the synthesis of detectable p85 or in any alteration of the cytoplast morphology or microtubule complex, suggesting that the temperature-sensitive lesion affects some event occurring in the nucleus.

Purified low-molecular-weight protein kinase from murine sarcoma virus particles catalyzes tyrosine phosphorylation endogenously but phosphorylates cellular proteins at serine

The low-molecular-weight (LMW) protein kinase associated with high-titer murine sarcoma virions have been extensively purified by ammonium sulfate fractionation. Bio-Gel P-100 gel filtration, DEAE-cellulose and carboxymethyl cellulose chromatography. The purified enzyme migrates as a 16K polypeptide in polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The enzyme catalyzes phosphotransfer with ATP as a phosphate donor to various exogenously added proteins as acceptors; it requires Mg2+ and is independent of cyclic AMP. The enzyme preparation catalyzes a low level of phosphorylation in the absence of any exogenously added substrate and forms phosphotyrosine. However, in the presence of acceptor protein molecules including total soluble cytoplasmic proteins of murine sarcoma virus-transformed mouse cells, the phosphorylated end products contain predominantly phosphoserine. The virion-associated enzyme also shows a preference for phosphorylating certain polypeptides in the soluble cytoplasmic extracts of murine sarcoma virus-transformed cells.