Guanine nucleotide-binding proteins that enhance choleragen ADP-ribosyltransferase activity: nucleotide and deduced amino acid sequence of an ADP-ribosylation factor cDNA.

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RESUMO

Three (two soluble and one membrane) guanine nucleotide-binding proteins (G proteins) that enhance ADP-ribosylation of the Gs alpha stimulatory subunit of the adenylyl cyclase (EC 4.6.1.1) complex by choleragen have recently been purified from bovine brain. To further define the structure and function of these ADP-ribosylation factors (ARFs), we isolated a cDNA clone (lambda ARF2B) from a bovine retinal library by screening with a mixed heptadecanucleotide probe whose sequence was based on the partial amino acid sequence of one of the soluble ARFs from bovine brain. Comparison of the deduced amino acid sequence of lambda ARF2B with sequences of peptides from the ARF protein (total of 60 amino acids) revealed only two differences. Whether these are cloning artifacts or reflect the existence of more than one ARF protein remains to be determined. Deduced amino acid sequences of ARF, Go alpha (the alpha subunit of a G protein that may be involved in regulation of ion fluxes), and c-Ha-ras gene product p21 show similarities in regions believed to be involved in guanine nucleotide binding and GTP hydrolysis. ARF apparently lacks a site analogous to that ADP-ribosylated by choleragen in G-protein alpha subunits. Although both the ARF proteins and the alpha subunits bind guanine nucleotides and serve as choleragen substrates, they must interact with the toxin A1 peptide in different ways. In addition to serving as an ADP-ribose acceptor, ARF interacts with the toxin in a manner that modifies its catalytic properties.

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