- Volume 52, Issue 1, 1981

A glycoprotein with an approx. mol. wt. of 55000, encoded by spleen focus-forming virus (SFFV), was detected with an antiserum against xenotropic murine leukaemia virus (MuLV) in various cell lines infected with the Friend strain of SFFV, suggesting that the SFFV-specific RNA coding for gp55 is related to the xenotropic virus. The proteins from three cell lines differed from each other in molecular size and gave different peptide maps. The differences in the gp55 molecules were shown to depend on differences in the SFFV genome, but not on differences in glycosylation or processing of the precursor polypeptides of gp55 in the different cells. These results indicate virus strain specificity of SFFV in gp55 formation.

Enhancement of endogenous xenotropic virus expression has been found upon treatment with tetrapeptides of a high-passage clone of Balb/c (K-Balb) mouse cells transformed with Kirsten sarcoma virus. Tuftsin (Thr-Lys-Pro-Arg) and kentsin (Thr-Pro-Arg-Lys) increased the expression of virus that was infectious for rat, but not mouse, cells in a concentration-dependent fashion. The enhancement of virus expression by the tetrapeptides was proportional to the spontaneous release of virus. The infectivity of the enhanced virus was neutralized by goat anti-RLV gp70 serum. Actinomycin D inhibited the induction of virus, suggesting that enhanced expression required de novo RNA synthesis. The effects observed using K-Balb cells offer an opportunity to study the many biological effects of these peptides in a fibroblast culture system.

The addition of salts of the rare earth elements (lanthanides) enhances by a factor of two or more the initial activity of human fibroblast, leukocyte and immune interferons as well as mouse L-cell interferon. Furthermore, the salts of the lanthanides (0.01 to 0.002 m) preserve the activity of human fibroblast interferon not only after 4 days of heating at 37 °C but also after exposure to severe shearing forces; human leukocyte interferon was stabilized to a lesser degree, albeit significantly.

Treatment of K/Balb cells with IdUrd leads to the expression of endogenous virus and interferon is formed which subsequently can suppress the release of virus into the medium. Under these conditions, a protein kinase activity capable of phosphorylating an endogenous polypeptide of apparent mol. wt. 67000 is detected. This kinase activity is analogous to that induced in K/Balb cells treated with exogenous interferon. The induction of the protein kinase activity in IdUrd-treated K/Balb cells can be blocked by anti-interferon serum, thus providing evidence for the presence and action of interferon.