Determinants for the syncytium-inducing phenotype of HIV-1 subtype F isolates are located in the V3 region

The HIV-1 syncytium-inducing phenotype is determined by virus replication a nd the presence of cytopathic effects in MT-2 cells, There is a strong corr elation between the syncytium-inducing/MT-2-tropic phenotype and positively charged amino acids at positions 306 and 320 in the V3 loop for HIV-1 subt ypes A, B, D, and E, In contrast, a lack of correlation between signature a mino acids and syncytium formation in MT-2 cells for subtype F viruses from Romania has been reported, Virus phenotype and V3 loop amino acid sequence s from Romanian HIV-1 subtype F isolates were further investigated in the p resent study. While the determinants of MT-2 tropism are clearly harbored i n the V3 loop of subtype F isolates from Romania, the induction of syncytiu m formation occurs in the presence or absence of positively charged amino a cids at positions 306, 320, and/or 324, However, the net positive charge of V3 loop sequences derived from syncytium-inducing viruses was higher than that of the nonsyncytium-inducing isolate.