Proper processing of avian sarcoma/leukosis virus capsid proteins is required for infectivity

The formation of the mature carboxyl terminus of CA in avian sarcoma/leukem ia virus is the result of a sequence of cleavage events at three PR sites t hat lie between CA and NC in the Gag polyprotein. The initial cleavage form s the amino terminus of the NC protein and releases an immature CA, named C AI, with a spacer peptide at its carboxyl terminus. Cleavage of either 9 or 12 amino acids from the carboxyl terminus creates two mature CA species, n amed CA2 and CA3, that can be detected in avian sarcoma/leukemia virus (R, B, Pepinsky, I. A. Papayannopoulos, E. P, Chow, N. K. Krishna, R. C. Craven , and V. M. Vogt, J. Virol, 69:6430-6438, 1995), To study the importance of each of the three CA proteins, we introduced amino acid substitutions into each CA cleavage junction and studied their effects on CA processing as we ll as virus assembly and infectivity, Preventing cleavage at any of the thr ee sites produced noninfectious virus, In contrast, a mutant in which cleav age at site 1 was enhanced so that particles contained CA2 and CA3 but litt le detectable CA1 was infectious. These results support the idea that infec tivity of the virus is closely linked to proper processing of the carboxyl terminus to form two mature CA proteins.