THE HIV-1 REGULATORY PROTEIN NEF HAS A SPECIFIC FUNCTION IN VIRAL EXPRESSION IN A MURINE MACROPHAGE CELL-LINE

Expression of reporter genes under the control of the HIV-1 long termi nal repeat (LTR) was up-regulated in the murine macrophage cell line R AW264 by cotransfection of a plasmid coding for the viral regulatory p rotein Nef. To determine if a discrete section of the LTR was exclusiv ely responsive to Nef, a series of promoters was produced by successiv e 5' deletions from the LTR up to the boundary of the enhancer region. These truncated promoters were as active as the full-length sequence in the RAW264 cells, but elimination of the direct repeats and one of the three Sp1 sites reduced promoter activity to minimal levels. Trans cription driven by all constructs was equally susceptible to the trans -activating effect of Nef and could be increased further by the additi on of a Tat-expressing plasmid to the cotransfection. Open reading fra mes of nef from NE4-3, from HXB2, which has a premature stop, and a fu lly functional hybrid of the two under the control of the SR alpha art ificial promoter (SV40 early promoter plus HTLV-I R-U5') were able to transactivate the LTR in RAW264 cells to the same degree as HXB3 nef u nder the control of the cytomegalovirus (CMV) immediate-early promoter . A frameshift mutation of Nef at the XhoI site at position 8475 did n ot abrogate trans-activation of the LTR in macrophages. To further def ine the effective trans-activation region of Nef, internal deletions w ere made. Changes downstream of the XhoI site at amino acid 35 resulte d in little or no reduction in trans-activation, whereas a deletion be tween the CMV promoter of the expression plasmid and the XhoI site lar gely abolished activity. Nef trans-activation of the LTR may be restri cted to macrophages. Parallel cotransfection experiments in COS-1 simi an fibroblast-like cells showed repression of reporter expression by N ef. Results suggested that the section of nef responsible for transact ivation of the LTR in macrophages differed slightly from that sufficie nt for trans-repression in fibroblasts. Translation of the protein fro m the first translation start site (Met-1) rather than from the second in-frame ATG (Met-20) appears to be necessary for the trans-activatin g effect of Nef in RAW264 cells. Mutation of the initial ATG to ATA le d to loss of trans-activating activity. Expression of Nef also has a c ytostatic/cytotoxic effect on RAW264 cells indicated by a reduced rate of establishment of stably transfected clones. The cytostatic effect of Nef was not relieved by internal deletions in the coding sequence. The effect required transcription of one of two regions of the nef gen e. These observations suggest that effects of Nef on cell growth and L TR-driven transcription in RAW264 cells are not related. Trans-activat ion of the HIV-1 LTR in the RAW264 cells by the regulatory protein Nef may be due to a novel protein-promoter interaction.