A 30-amino-acid segment of C/EBP, a newly discovered enhancer binding protein, shares notable sequence similarity with a segment of the cellular Myc transforming protein. Display of these respective amino acid sequences on an idealized alpha helix revealed a periodic repetition of leucine residues at every seventh position over a distance covering eight helical turns. The periodic array of at least four leucines was also noted in the sequences of the Fos and Jun transforming proteins, as well as that of the yeast gene regulatory protein, GCN4. The polypeptide segments containing these periodic arrays of leucine residues are proposed to exist in an alpha-helical conformation, and the leucine side chains extending from one alpha helix interdigitate with those displayed from a similar alpha helix of a second polypeptide, facilitating dimerization. This hypothetical structure is referred to as the "leucine zipper," and it may represent a characteristic property of a new category of DNA binding proteins.

In two previous studies we described the properties of a heat-stable DNA-binding protein present in rat liver nuclei. This protein, hereafter termed C/EBP, is capable of selective binding to the CCAAT homology of several viral promoters (Graves et al. 1986), as well as the core homology common to many viral enhancers (Johnson et al. 1987). We now report the isolation of a recombinant clone of the gene that encodes C/EBP. Expression of the clone in bacterial cells yields a protein that binds in vitro to both the CCAAT homology and the enhancer core homology, providing conclusive evidence that a single gene product accounts for both binding activities. By examining the properties of protease-derived fragments of C/EBP, we have localized its DNA-binding domain to a 14-kD fragment. A 60-amino-acid segment located within the DNA-binding domain of C/EBP bears sequence similarity to the products of the myc and fos oncogenes.

The receptor for the wilt-inducing phytotoxin fusicoccin was purified to homogeneity from plasma membranes of Commelina communis as a complex with the radioligand [3H]9'-nor-8'-hydroxyfusicoccin. The preparation consisted of two polypeptides with apparent molecular masses of 30.5 kDa and 31.5 kDa and with isoelectric points of around pH 5.2 and 5.3, respectively. The proteins were N-terminally blocked. Internal amino acid sequences were obtained for both polypeptides of the fusicoccin-binding complex. Sequence information, as well as subsequent immunological analysis, proved that both polypeptides are members of the eukaryotic 143-3 family, which comprises structurally conserved regulatory proteins of widespread occurrence and a wide range of functions. 143-3 isoform(s) constituting the fusicoccin receptor are distinguishable from other cellular 143-3 proteins by their tight association with the plasma membrane. Applying temperature-induced Triton X-l 14 phase separation experiments, they, as well as the target enzyme of fusicoccin action, the H'-ATPase, partitioned into the phospholipid-rich fraction which contains the most hydrophobic proteins. The results discussed herein provide a basis for the elucidation of the molecular mechanism of fusicoccin action.