LYSIS OF ONCORNAVIRUSES BY H U M A N SERUM Isolation of the Viral Complement ( C 1 ) Receptor and Identification as pl 5 E

Human serum inactivates RNA tumor viruses as Welsh and co-workers (1-4) have demonstrated. The mechanism of inactivation was shown to be complement-mediated viral lysis, not requiring virion-specific antibodies. All C-type RNA viruses were lysed by sera from primates but not from rabbits, mice, guinea pigs, or goats. Cooper et al. (4) showed that integrity of the classical complement sequence was an absolute requirement for lysis and that, in absence of specific antibody, Clq I subserved the virus recognition function. The antibody independent nature of this virolytic reaction suggests that complement plays an important role in natural or nonspecific defense against C-type RNA viruses. We have therefore explored the envelope proteins of Moloney leukemia virus as to their capacity to activate complement, and have identified a single, low molecular weight surface protein as the viral receptor and activator of C1.