Mammalian atria contain peptides that promote the excretion of salt and water from the kidney. When rat atrial tissue is extracted under conditions known to inhibit proteolysis, four natriuretic peptides, cardionatrins I to IV, are consistently isolated. These peptides derive from a common precursor, preprocardionatrin, of 152 amino acids, whose sequence was determined by DNA sequencing of a complementary DNA clone. Amino acid sequencing located the start points of cardionatrins I, III, and IV in the overall sequence. Cardionatrin IV most closely resembles procardionatrin because it begins immediately after the signal sequence at residue 25. Cardionatrin III begins at residue 73, and cardionatrin I, sequenced previously, begins at residue 123. Compositional analysis indicated that each of these cardionatrins extends up to tyrosine at position 150 but lacks the terminal two arginine residues.

We used a Z-DNA affimity column to isolate a collection of Z-DNA binding proteins from a high salt extract of Escherichia cohl. We identified one of the major Z-DNA binding proteins of this fraction, not as a protein involved in gene regulation or genetic recombination, but rather as an outer membrane porin protein. We then showed that several other known phospholipid-binding proteins (bovine lung annexins and human serum lipoproteins) also bind much more tightly to Z-DNA than to B-DNA. In all cases, this Z-DNA binding was strongly blocked by competition with acidic phospholipids, such as cardiolipin. Our results raise the question whether many of the Z-DNA binding proteins previously isolated are actually phospholipid-binding proteins.The biological significance of left-handed Z-DNA remains unknown. Although appropriate regions of natural DNA can be made to adopt a left-handed conformation in vitro (1-5), which can then influence a number of biochemical processes (6-11), most evidence shows that similar effects do not occur in vivo (10-13). On the other hand, proteins that clearly prefer to bind to Z-DNA rather than to B-DNA have been isolated from a number of natural sources (14-23). Although the identity and functions of these proteins are largely unknown, the existence of such proteins has been taken as evidence for a biological function of Z-DNA.In the present paper, we consider whether the existence of Z-DNA binding proteins indeed provides support for the natural existence of Z-DNA. Our concerns arise because of our recent and quite unexpected finding that yolk proteins from nematodes, chickens, and frogs bind strongly and preferentially to Z-DNA (24). Although yolk proteins have a higher Z-DNA specificity than most of the Z-DNA binding proteins previously isolated, it is unlikely that they bind to Z-DNA in vivo. Thus, if we had not happened to identify these proteins, they would have been accepted as bona fide Z-DNA binding proteins, with some yet to be defined biological function. We thus became curious about the true identity of Z-DNA binding proteins isolated purely by physical criteria. In particular, since yolk proteins appear to bind Z-DNA at a phospholipid-binding site (24), we became suspicious that at least some of the previously isolated Z-DNA binding proteins might actually be phospholipid-binding proteins.In the present paper, we used the Z-DNA affinity column developed by Rich and coworkers (14,16,17,21,23) to isolate Z-DNA binding proteins from a high salt extract of Escherichia coli. We identified one of the most prominent of these proteins as an outer membrane porin protein, which is unlikely to be involved in either gene regulation or genetic recombination. We then show that phospholipid-binding proteins as diverse as bovine lung annexins and human serum lipoproteins all bind strongly and preferentially to Z-DNA. We feel that our results raise the distinct possibility that many of the uncharacterized Z-DNA binding proteins previously isolated are actually phospholipid-binding pro...

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