Structure and function of HIV-1 and SIV Tat proteins based on carboxy-terminal truncations, chimeric Tat constructs, and NMR modeling

To further define the structure and function of the domains in HIV-1 and SI V Tar proteins, chimeric Tar cDNA expression constructs were generated with crossover points at the carboxy-terminal end of the cysteine rich domain. The chimera containing the amino-terminal region of SIV and carboxy-termina l region of HIV exhibited activity similar to HIV-1 Tar and SIV Tat on both the HIV-1 and SIV LTRs. In contrast, the reciprocal chimera functioned poo rly. As determined by the activity of carboxy-terminal truncation mutants, the region immediately downstream of the basic domain is critical for effic ient transactivation by HIV-1 Tat, but not SIV Tat protein. In this report, we present a model for Tat domains based on NMR data and the known functio nal properties of Tat protein. According to our modeling two sites for prot ein:protein interactions are present in HIV-1 and SIV Tat proteins. Site I, which is presumably involved in cyclin T binding, is similar in both HIV-1 and SIV Tat proteins as well as in Tat chimeras. Site II, however appears structurally different in HIV-1 and SIV Tat models, although in both cases is comprised of amino and carboxy-terminal residues. Differences in Site II may thus account for the differential activities of HIV-1 and SIV Tat carb oxy-terminal truncations. Site II in the poorly active chimera differs sign ificantly from that found in HIV-1 and SIV Tat proteins. The two site struc tural model presented here may have important implications for the role of Tat in HIV pathogenesis and may provide insights for the design of Tat vacc ines and targeted therapeutics. (C) 1998, Elsevier, Paris.