Characterization of human immunodeficiency virus type 1 nef gene products translated in vitro and expressed in mammalian cells

Expression of the human immunodeficiency virus type 1 nef gene was studied by in vitro transcription-translation and by transfection into monkey COS cells. Two Nef-related peptides, of 27 and 25 kDa, were identified by immunoprecipitation with anti-Nef antibodies. The relation between these two proteins was determined by metabolically labeling transfected COS cells and by deleting the initiator methionine of nef. We found that the 25-kDa polypeptide is not a cleavage product of 27-kDa Nef but rather is initiated from an internal ATG 57 bases downstream from the Nef initiation site. Myristoylation of the 27-kDa but not of the 25-kDa Nef was demonstrated by the contranslational modification of Nef in an in vitro reticulocyte translation system. The myristoylation pattern of the two Nef polypeptides further implies that the 25-kDa polypeptide lacks the amino terminus of 27-kDa Nef. Cellular localization of the various forms of Nef was studied in transiently transfected COS cells. Myristoylation was found to be necessary for membrane association of Nef. Myristoylation-deficient 27-kDa Nef mutant and 25-kDa Nef were confined to the soluble cytoplasmic fraction of transfected cells, whereas part of the wild-type 27-kDa Nef was membrane attached.