The attenuated V60 strain of channel catfish virus possesses a deletion in ORF50 coding for a potentially secreted glycoprotein.

A wild-type strain of channel catfish virus was compared at the genomic level with the attenuated strain V60. In addition to several minor differences, restriction mapping revealed one major deletion (approximately 1200 bp) in ORF50 of the V60 strain. Cloning and sequencing of part of this ORF confirmed the presence of a 1164-bp deletion. It should result in a protein of 282 amino acids instead of 670. The predicted truncated protein lacks most of a threonine-rich, highly repetitive region in its central part. Since the protein encoded by ORF50 possesses a hydrophobic N-terminal leader sequence and no membrane anchor sequence, we suggest that it could be a secreted glycoprotein. This protein might be N-glycosylated (35 potential sites) and, given the repetitive arrangement of its residues (mainly threonines), also heavily O-glycosylated like the mucin-type glycoproteins. The deletion observed in ORF50 of the V60 strain implies the loss of 24 potential N-glycosylation sites and should considerably reduce the extent of O-glycosylation.