EFFECTS OF MUTATIONS IN CONSTANT REGION-3 AND REGION-4 OF ENVELOPE OFSIMIAN IMMUNODEFICIENCY VIRUS

Twenty-six mutant forms of simian immunodeficiency virus strain mac239 were constructed with changes in constant region 4 (C4) of env. Twent y-four of these had a single amino acid change, one had changes in two amino acids, and one had a deletion of eight amino acids. The effects of these mutations on viral replication, gp160 processing, and bindin g of env protein to soluble CD4 receptor were analyzed. The C4 region was relatively sensitive to sequence changes since only 11 of the 26 m utants replicated appreciably. Eight of the 15 mutants that were repli cation incompetent exhibited grossly defective processing of the gp160 env precursor; these mutations likely resulted in global effects on g p160 structure. Six of the replication incompetent mutants exhibited n ormal or near normal gp160 processing and binding of env protein to sC D4 and thus were probably blocked at some step subsequent to binding o f virus to its CD4 receptor. Only one of the C4 mutations, 441W --> R, resulted in greatly decreased binding to sCD4 while retaining normal processing of gp160. The equivalent residue in HIV-I has similarly bee n shown previously to be important for binding of HIV-1 to the CD4 rec eptor. Since a W --> S mutation at position 441 in C4 of SIVmac239 aff ected both gp160 processing and sCD4 binding, it is not clear whether the 441 tryptophan is actually important for contacting CD4 or for mai ntaining an appropriate configuration. Mutations within a highly conse rved GGDPE sequence in Os of SIVmac239 specifically affected CD4 bindi ng, which is also similar to previous findings with HIV-1. These resul ts demonstrate similar sequence requirements in SIVmac and HIV-1 env f or binding CD4, but they raise doubts as to whether C4 sequences are d irectly involved in the binding. (C) 1995 Academic Press, Inc.