Immunochemical methods were used to examine the effect of
viral infection on the dynamics of intracellular
ubiquitin pools.
Infection of either the human lung
carcinoma line A-549 or the mouse fibroblast line L929 with encephalomyocarditis virus had little effect on either the distribution or fractional level of intracellular
ubiquitin conjugates. In contrast,
viral infection resulted in a significant decline in the steady state content of the mono-
ubiquitin conjugate to
histone 2A (
uH2A). Prior treatment with
interferons protected against this decrease of
uH2A. Furthermore,
interferons induced the de novo synthesis of a 15-kDa
protein immunologically related to
ubiquitin. The
ubiquitin cross-reactive
protein (UCRP) was not constitutively present in control cells but was significantly induced in various cells sensitive to the
biological effects of
interferons. Induction of UCRP with respect to both time and
interferon concentration dependence closely paralleled the appearance of resistance to
viral infection and could be blocked by low levels of
actinomycin D. Subsequent studies demonstrated that UCRP was identical to an
interferon-induced 15-kDa
protein whose sequence has recently been reported (Blomstrom, D. C., Fahey, D., Kutny, R., Korant, B. D., and Knight, E. (1986) J. Biol. Chem. 261, 8811-8816). An authentic sample of the 15-kDa
protein was found to co-migrate with UCRP and to cross-react with two different anti-
ubiquitin antibodies. Using the authentic 15-kDa
protein as a standard, UCRP accumulated to 6.2 +/- 0.5 pmol/10(6) cells and 34 +/- 2 pmol/10(6) cells in
interferon-treated A-549 and L929 cultures, respectively. Comparison of the primary sequence of the 15-kDa
protein to that of
ubiquitin indicated that the former is composed of two domains, each of which bears striking homology to
ubiquitin. These observations suggest that the 15-kDa
protein may represent one example of a functionally distinct family of
ubiquitin-like proteins.