essential regions for antiviral activities of actinohivin a sugarbinding antihuman immunodeficiency virus protein from an actinomycete actinohivin ah is a potent antihuman immunodeficiency virus hiv protein that consists of highly conserved threetandem repeats segments 1 2 and 3 the molecular target of ah in its antihiv activity is highmannosetype saccharide chains of hiv gp120 this article deals with sequence requirements for the antihiv activity of ah the deleted or substituted dnas encoding ah or hisah were prepared using mutagenic oligonucleotide primers in pcr the mutant constructs were expressed in escherichia coli and the activities of the recombinant protein products were examined by a syncytiumformation assay system that mimics antihiv activity the single segment mutant hisahs showed no antisyncytiumformation activity but the mutant hisahs which consists of 2 or 3 segments retained reduced activities hisah6114 dramatically reduced the antisyncytiumformation activity to that of hisah36114 or hisahi5a furthermore hisahq33a hisahq71a and hisahq109a in which glutamine residues were substituted into alanine showed reduced activities of 120 110 and 130 respectively in antisyncytium formation compared with hisah these results indicate that three segments of ah are necessary for potent antisyncytiumformation activitythat is for potent antihiv activity and the cooperated involvement of each segment of ah increased the ahgp120 interaction