The c-rel proto-oncogene encodes a 75-kDa protein (p75cI) which is present in the cytosol of chick embryo fibroblasts (CEF) associated with a distinct set of cellular proteins with molecular masses of 40, 115, and 124 kDa. CEF cultures arrested in S phase of the cell cycle, or enriched for G2 or mitotic cells, were examined to determine whether the expression of c-rel was altered during the cell cycle. Levels of p75C`remained constant in all portions of the cell cycle examined; however, a Rel-related protein with an apparent molecular mass of 64 kDa was detected in nuclei of S-phase cells. As cells enter G2, the level of this protein in the nucleus decreases. This protein reacts with antiserum generated against the carboxy terminus of p75C" in radioimmunoprecipitations and Western immunoblot experiments and was also detected in a Western immunoblot with antiserum generated against the first 161 amino acids of pp59v". Thus, unlike other Rel/NF-KB family members, p64 has carboxy-terminal homology with c-Rel. The majority of peptides generated by partial proteolytic cleavage of p64 are shared with peptides generated by digestion of p75C and/or pp59v'. We suggest that this protein represents a new member of the Rel family of transcription factors and is located in the nucleus of avian fibroblasts during S phase of the cell cycle.The proto-oncogene c-rel encodes a 75-kDa protein (p75C'-) which is a member of the Rel/NF-KB family of transcription factors (38,53). This family includes the product of the v-rel oncogene carried by reticuloendotheliosis virus (REV-T), the Drosophila melanogaster maternal morphogen Dorsal, the various subunits of the mammalian transcription complex NF-KB, human p49, and murine RelB and its human homolog I-Rel (24,34,42,(46)(47)(48)55). These proteins share homology within approximately 350 amino acids near the amino terminus (the Rel domain), and all except v-Rel and c-Rel are completely divergent at their carboxy termini (10,34,42,47,48). With one exception, these proteins are able to bind DNA in vitro to the consensus NF-KB binding site (KB site) as homo-or heterodimers (4,6,34,42,44,47,48). Dorsal has not been shown to form heterodimers, nor has it been shown to bind the consensus KB site; however, it does bind a related site in the zen promoter (31). Recent data utilizing degenerate oligonucleotides indicate that different combinations of Rel family members preferentially bind divergent KB-related motifs (44). Which Rel family members heterodimerize in vivo and are physiologically active is still a matter of speculation.NF-KB was initially identified in lymphoid cells as a heterodimer composed of a 50-kDa subunit (p50) which is derived from a 110-kDa precursor by proteolytic cleavage and a 65-kDa subunit (p65) (2-4, 23, 25, 34, 58). This DNA-binding complex is sequestered in the cytosol by association with the inhibitory protein IKB (2, 3). p75C"' is found in the cytosol of avian lymphocytes associated with three major proteins (40, 115, and 124 kDa) (15,19,35,40). The 40-kDa protein ...
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