Regulation of pp60c-src and its interaction with polyomavirus middle T antigen in insect cells

High yields of soluble, biologically active pp60c-src and middle t antigen (MTAg) of polyomavirus were produced in insect cells, using a baculovirus expression system. In mammalian cells, pp60c-src undergoes a regulatory phosphorylation on Tyr-527 in vivo and is autophosphorylated on Tyr-416 in vitro. In insect cells, pp60c-src was phosphorylated primarily on Tyr-416, although Tyr-527 was detectable at a low level. A kinase-negative mutant of pp60c-src was not phosphorylated on either Tyr-527 or Tyr-416 in insect cells and thus is an excellent biochemical reagent to search for the regulatory kinase that usually phosphorylates Tyr-527 in mammalian cells. MTAg synthesized in insect cells was not phosphorylated on tyrosine residues in vivo or in vitro, suggesting that it did not associate with any endogenous tyrosine kinases. However, MTAg isolated from cells coinfected with viruses encoding both MTAg and pp60c-src was phosphorylated on tyrosine residues both in vivo and in vitro.

The product of the retinoblastoma susceptibility gene has properties of a cell cycle regulatory element

The retinoblastoma susceptibility gene product, Rb, is suspected to suppress cell growth. Rb is a 110-114 kd nuclear phosphoprotein. We have previously demonstrated that SV40 T antigen binds only to unphosphorylated Rb, and not pp112-114Rb, the family of phosphorylated Rb. Here we demonstrate the cell cycle-dependent phosphorylation of Rb. In G0/G1 cells, virtually all the Rb is unphosphorylated. In contrast, during S and G2, it is largely, if not exclusively, phosphorylated. Rb phosphorylation occurs at the G1/S boundary in several cell types tested. A 14 residue peptide, corresponding to the SV40 T domain required for transformation, is able to compete effectively with SV40 T for binding to p110Rb. We propose a model to explain how Rb may suppress cell growth by acting as a cell cycle regulatory element.

Human cdc2 protein kinase is a major cell-cycle regulated tyrosine kinase substrate

cdc2 is a catalytic subunit of a protein kinase complex, called the M-phase promoting factor, that induces entry into mitosis and is universal among eukaryotes. In HeLa cells, cdc2 is shown to be the most abundant phosphotyrosine-containing protein and its phosphotyrosine content is subject to cell-cycle regulation. One site of cdc2 tyrosine phosphorylation in vivo is selectively phosphorylated by pp60c-src in vitro.

The carboxy terminus of pp60c-src is a regulatory domain and is involved in complex formation with the middle-T antigen of polyomavirus

A large number of mutations were introduced into the carboxy-terminal domain of pp60c-src. The level of phosphorylation on Tyr-416 and Tyr-527, the transforming activity (as measured by focus formation on NIH 3T3 cells), kinase activity, and the ability of the mutant pp60c-src to associate with the middle-T antigen of polyomavirus were examined. The results indicate that Tyr-527 is a major carboxy-terminal element responsible for regulating pp60c-src in vivo. A good but not perfect correlation exists between lack of phosphorylation at Tyr-527 and increased phosphorylation at Tyr-416, between elevated phosphorylation on Tyr-416 and activated kinase activity, and between activated kinase activity and transforming activity. Phosphorylation of Tyr-527 was insensitive to the mutation of adjacent residues, indicating that the primary sequence only has a minor role in recognition by kinases or phosphatases which regulate it in vivo. Three mutants which have in common a modified Glu-524 residue were phosphorylated on Tyr-416 and Tyr-527 and were weakly transforming. This suggests that other mechanisms besides complete dephosphorylation of Tyr-527 can lead to increased phosphorylation of Tyr-416 and activation of the transforming activity of pp60c-src. Furthermore, the residues between Asp-518 and Pro-525 were required to form a stable complex with middle-T antigen. The proximity of these sequences to Tyr-527 suggests a model in which middle-T activates pp60c-src by binding directly to this region of the molecular and thereby preventing phosphorylation of Tyr-527. Alternatively, middle-T binding may mediate a conformational change in this region, which in turn induces an alteration in the level of phosphorylation at Tyr-527 and Tyr-416.

Tyrosine phosphorylation regulates the biochemical and biological properties of pp60c-src

To investigate the importance of tyrosine phosphorylation in the regulation of pp60c-src, we have substituted phenylalanine for tyrosine at positions 416, 519, and 527. Cells expressing the 527 or the 519/527 mutant but not the 416 or the 519 mutant were morphologically transformed, grew in soft agar, and formed foci. In addition, the 527 and 519/527 mutants had elevated kinase activities in vitro. Modifying Tyr 416 to phenylalanine in the 527 or the 519/527 mutants only partially inhibited their kinase activities yet abolished their ability to induce focus formation and promote growth in soft agar. These results suggest that two events must occur to activate the full transforming potential of pp60c-src: hypophosphorylation at Tyr 527 and hyperphosphorylation at Tyr 416.

Differentiation of myeloid cells is accompanied by increased levels of pp60c-src protein and kinase activity

We have detected a significant increase in the levels of pp60c-src kinase activity associated with the differentiation of myeloid cell lines HL-60 and U-937. The induction of pp60c-src kinase activity becomes apparent approximately 14 hr after the addition of phorbol 12-myristate 13-acetate and increases 20-fold by 72 hr. The enhanced kinase activity can be accounted for by elevated levels of c-src protein in the differentiated cells. When nonleukemic bone marrow cells were examined, myeloid progenitor cells exhibited a low level of pp60c-src kinase activity. As these cells are allowed to differentiate in culture, the resulting adherent monocytes are as high in pp60c-src kinase activity as HL-60 cells induced to differentiate into monocytes. A strong correlation is found between the levels of pp60c-src kinase activity and the degree of monocytic differentiation of the cells from patients with acute myeloid leukemia. Our findings suggest that the activation of pp60c-src kinase activity is a normal physiological event associated with myeloid differentiation.

Retrovirus shuttle vector for study of kinase activities of pp60c-src synthesized in vitro and overproduced in vivo

We have constructed a recombinant murine retrovirus which efficiently transduces avian pp60c-src into murine cells and which is easily rescued from infected cells in plasmid form. To characterize the virus, several randomly selected NIH 3T3 lines were isolated after infection with recombinant retroviral stocks. All lines overproduced avian pp60c-src and appeared morphologically normal. Immunoprecipitates made from these lines with antisera specific for pp60c-src were tested for their kinase activities in vitro. We find that both autokinase and enolase kinase activities increase proportionately with the level of pp60c-src in the immunoprecipitates. To further test the authenticity of the pp60c-src encoded by the retroviral vector, these analyses were repeated in the presence of polyomavirus middle T antigen. Avian pp60c-src was activated as a protein kinase, indicating that the virally encoded pp60c-src interacts normally with middle T antigen. Interestingly, by increasing the intracellular levels of pp60c-src 15-fold over normal endogenous levels, we were unable to obtain a proportionate increase in the amount of middle-T-antigen-pp60c-src complex. Finally, using the shuttle features designed into the vector, we have isolated the first fully processed cDNA encoding functional avian pp60c-src X pp60c-src synthesized in vitro with this cDNA had intrinsic protein kinase activity and no detectable phosphatidylinositol kinase activity.

Replication of the vesicular stomatitis virus genome in permissive and nonpermissive host cells

Permissive infections of BHK cells and nonpermissive infections of Raji cells were probed for the accumulation of vesicular stomatitis virus intracellular RNAs. In Raji cells, the onset of vesicular stomatitis virus transcription and replication was delayed when compared to BHK cells, and the accumulation of plus and minus sense leader RNAs was significantly reduced. In contrast, full length plus and minus strand replicative RNAs accumulated in Raji cells to levels approximately equivalent to those in BHK cells. In both cell types, approximately four times as many minus strands as plus strands were detected late in the infections. At 16 h postinfection, 12% of the total genomic RNA synthesized in BHK cells was packaged and released whereas only 0.8% was released from Raji cells. In addition, of those particles released by Raji cells, only 1% were infectious whereas 77% of those released by BHK cells were infectious. The virions released from both cell types contained similar amounts of the five viral proteins, however. Analysis of virions from Raji cells revealed a faster electrophoretic mobility of the glycoprotein than the glycoprotein in virions released from BHK cells. These results suggest that Raji cells may be restricted in their ability to support a complete infection at the level of virus assembly rather than at the level of RNA replication.

Sequential synthesis of small capped RNA transcripts in vitro by vesicular stomatitis virus

Using purified viral or intracellular transcriptive complexes (RNP cores) of vesicular stomatitis virus (VSV), we have identified several small RNA species, ranging in size from 12 to 47 nucleotides in length that are synthesized in vitro by the genomic RNA. One group of small RNA transcripts is composed of three species that are capped at their 5' termini. Two of the capped species are from the start of the N gene and one is from the start of the NS gene. Unlike the previously described 5' triphosphated small RNAs, the templates encoding these small capped RNAs had uv target sizes greater than their respective lengths. In addition, these RNAs appeared sequentially during synchronized in vitro transcription reactions. Thus, these results provide evidence that sequences representing the 5'-capped termini of N and NS mRNAs are synthesized concomitantly with their respective mRNAs rather than simultaneously at the onset of transcription as proposed for the multiple entry, start-stop model (D. Testa, P. K. Chanda, and A. K. Banerjee, 1980, Cell 21, pp. 267-275). Together with the inability of the internally initiated 5'-triphosphated RNAs to be chased into mRNA (R. A. Lazzarini, I. Chien, F. Yang, and J. D. Keene, 1982, J. Gen. Virol. 58, 429-441), these results support a single entry model of VSV mRNA transcription.

Rapid and transient localization of the leader RNA of vesicular stomatitis virus in the nuclei of infected cells

The leader RNA transcript from the 3' end of the genome of vesicular stomatitis virus (VSV) has been detected in both the nucleus and cytoplasm of infected baby hamster kidney (BHK) cells. In the cytoplasm, leader RNA accumulated gradually throughout the infection to about 200 molecules per cell at 6 hr after infection. In the nucleus, however, there was a sharp and rapid increase in the concentration of leader RNA to approximately equal to 300 molecules per cell at about 2 hr after infection that decreased rapidly by 3 hr. This report presents evidence for nuclear localization of transcription products of a (-)-strand RNA virus other than influenza and supports the hypothesis that the leader RNA plays a role in the shutoff of host cell transcription.