Synthesis, transport, and morphogenesis of type adenovirus capsid proteins

During the period between 20 and 24 hr after infection of KB cells with type 5 adenovirus, at a time when approximately 85% of the proteins made were virus-specific, viral proteins were synthesized on polyribosomes with an average sedimentation coefficient of 200S. The polypeptide chains synthesized during a 1-min period of labeling with (14)C-amino acids had an average sedimentation coefficient of 3.4S in sucrose gradients containing 1% sodium dodecyl sulfate. Within 1 min after completion, the newly made polypeptide chains were released from polyribosomes, and the majority were transported into the nuclei within 6 min. Meanwhile, the immunological reactivity of the newly synthesized proteins also increased rapidly. During the same 6-min interval after synthesis, the single polypeptide chains assembled into multimeric proteins with average sedimentation coefficients of 6S, 9S, and 12S. The 6S and 12S proteins were identified immunologically as the fiber and hexon capsid proteins, respectively. The 9S protein was trypsin-sensitive and appeared to be the precursor of the penton; it was tentatively identified as the penton base. The penton had a sedimentation coefficient of about 10.5S and sedimented with the hexon in sucrose gradients. The concomitant migration of nascent proteins into the nuclei, development of the capsid proteins' immunological reactivity, and morphogenesis of the multimeric capsid proteins suggest that the single polypeptide chains or small complexes were transported into the nuclei where they assembled into mature structural proteins of the virion.