The polypeptides of adenovirus. III. Synthesis in infected cells

Eight structural polypeptides of adenovirus type 2 can be identified in the infected HeLa cell. All are stable; none is a precursor of another. The various polypeptides are synthesized at very different rates, which do not reflect the ratio in which they are represented in the virion itself. For example, the penton-base and fiber polypeptides are made in considerable excess, whereas the three polypeptides comprising the inner viral core are in relatively short supply. At least 80% of the viral protein found in infected cells is in the form of free hexons, pentons, and fibers, the polypeptides of which have been identified. Host protein synthesis proceeds completely normally until after viral capsid protein synthesis begins, then declines in parallel with the gradual rise in the rate of viral protein synthesis. If viral DNA replication is inhibited, no capsid proteins are made and no shutdown of cellular protein synthesis occurs; even when limited production of viral DNA and capsid protein is allowed, host cell protein synthesis is little affected even many hours later. This is consistent with the suggestion that the inhibition of cell protein synthesis may be attributed to competition for ribosomes between host and viral messenger RNA, rather than to any virus-coded "cell shutdown" protein. The viral messenger RNA molecules coding for each of the several structural proteins have approximately the same stability. In the presence of actinomycin D, they decay with a mean half-life of 6 hours.