Sequence homology between the structural polypeptides of minute virus of mice

Abstract

The DNA-containing (full) particles of minute virus of mice contain three polypeptide species, designated A (Mr=83,000), B (Mr=64,000) and C (Mr=61,000). These three proteins are compared here by tryptic and chymotryptic fingerprinting after radio-iodination of their tyrosyl residues in vitro. Polypeptide B and C digests are almost identical, thus confirming the precursor-product relationship between them suggested by previous kinetic studies. Both types of fingerprint show one peptide which occurs in the C polypeptide but not in B, indicating that the cleavage in vivo may not occur at either a tryptic or chymotryptic site. In addition to the relationship between B and C, all of the sequence of B is present in the largest polypeptide A, which constitutes≈16% of the total virion protein. The A polypeptide contains additional tyrosyl peptides, comprising about 20% of the total, which do not occur in either B or C. Proteolytic digestion of intact full particles in vitro shows that the cleavage of B in vivo can be closely mimicked by trypsin and to a lesser extent by chymotrypsin. However, the B polypeptide in the empty virion is resistant to cleavage by either enzyme in vitro, indicating that it adopts a different conformation in each particle type. This correlates well with the in vivo observation that empty particles contain polypeptide B, in addition to A, and do not contain any polypeptide C. The A polypeptide is completely resistant to cleavage by either enzyme in either particle, suggesting that the conformation of the common sequence in polypeptide A may be similar to that adopted by polypeptide B in empty virions. A scheme for the maturation of infectious parvovirus virions is described. © 1977 Academic Press Inc. (London) Limited.