PROTEIN SPECIES OF THE PARVOVIRUS MINUTE VIRUS OF MICE STRAIN MVMP - INVOLVEMENT OF PHOSPHORYLATED VP-2 SUBTYPES IN VIRAL MORPHOGENESIS

The pattern of induced protein species of the prototype strain of the parvovirus minute virus of mice was determined in permissive A9 mouse fibroblast cells by high-resolution two-dimensional gel electrophoresi s. Identities of the viral proteins in the gels were assigned by probi ng two-dimensional blots with antisera raised against either purified capsids (recognizing VP-1 and VP-2) or specific coding regions of the nonstructural proteins (NS-1 and NS-2) expressed as beta-galactosidase fusion products in bacteria. All viral proteins showed posttranslatio nal modifications, phosphate being a common substituent. The NS-1 prot ein migrated as a basic polypeptide in the pl range of 7.4 to 7.8 with multiple stages of modification and as a likely minor but hyperphosph orylated component in the neutral region of the gel. The NS-2 isoforms were resolved at a pl value close to 5.5 as three groups of unevenly phosphorylated polypeptides, each composed of at least two protein spe cies. Both VP-1 and VP-2 structural polypeptides were induced as heter ogeneous phosphoproteins. The major VP-2 protein could be resolved in the form of a consistent pattern of three abundant (a to c), two inter mediate (d and e), and one meager (f) neutral isoelectric focusing spe cies or subtypes. This posttranslational modification precedes and is uncoupled from viral assembly, and all of the VP-2 subtypes are packag ed into empty capsids at the induced stoichiometry. However, intracell ular full virions harbored additional phosphorylated subtypes (g to 1) and a subtle rearrangement in the whole VP-2 composition, while matur e virions purified from lysed cultures lacked these subtypes, coordina tely with the emergence of six neutral VP-3 subtypes. Thus, the virion coat undergoes a chemical transition entailed by genome encapsidation , in which phosphates seem to play a major role, triggering the prefer ential proteolytic cleavage of the more acidic VP-2 subtypes to VP-3. Parvoviruses, with small coding capacity, may regulate some morphogene tic steps, such as assembly, genome encapsidation, and maturation, by posttranslational modifications of their structural proteins.