THE CONSERVED, HYDROPHILIC AND ARGININE-RICH N-TERMINAL DOMAIN OF CUCUMOVIRUS COAT PROTEINS CONTRIBUTES TO THEIR ANOMALOUS ELECTROPHORETIC MOBILITIES IN SODIUM DODECYL-SULFATE-POLYACRYLAMIDE GELS

Although the M(r) values of the coat proteins (CPs) of several cucumov iruses have been calculated from their deduced amino acid sequences to be approximately 24,000, the experimentally determined M(r) values us ing the Laemmli SDS-PAGE system were 30,000-31,000. Examination of the amino acid composition revealed that these CPs are neither highly aci dic nor highly basic. Post-translational glycosylation or phosphorylat ion were also ruled out as contributing factors to the observed anomal ous electrophoretic mobility because the products of in vitro translat ion of cucumovirus RNA 4 and in vivo bacterial expression of the clone d CP gene co-migrated with authentic cucumovirus CPs. Comparison of th e hydropathy profiles of the CPs revealed the presence in each of a st rikingly similar, highly hydrophilic N-terminal domain of 30-32 amino acid residues that contains a cluster of basic amino acids, mainly arg inine. Selective chemical cleavage at tryptophan residues in the CPs o f cucumoviruses, known to contain single tryptophan residues, yielded two peptides; an N-terminal peptide that contained the conserved hydro philic domain and a C-terminal peptide. SDS-PAGE analysis showed that the N-terminal, but not the C-terminal, peptide exhibited the anomalou s electrophoretic mobility.