Effects of arfaptin 1 on guanine nucleotide-dependent activation of phospholipase D and cholera toxin by ADP-ribosylation factor.

Arfaptin 1, a approximately 39-kDa protein based on the deduced amino acid sequence, had been initially identified in a yeast two-hybrid screen using dominant active ARF3 (Q71L) as bait with an HL-60 cDNA library. It was suggested that arfaptin 1 may be involved in Golgi functions, since the FLAG-tagged protein was associated with Golgi membranes when expressed in COS-7 cells and could be bound to Golgi in vitro in an ADP-ribosylation factor (ARF)- and GTPgammaS-dependent, brefeldin A-inhibited fashion. Arfaptin 2, found in the same two-hybrid screen as arfaptin 1, is 60% identical in amino acid sequence and may or may not have an analogous function. We now report some effects of arfaptin 1 on ARF activation of phospholipase D and cholera toxin ADP-ribosyltransferase. Arfaptin 1 inhibited activation of both enzymes in a concentration-dependent manner and was without effect in the absence of ARF. Two ARF1 mutants that activated the toxin, one lacking 13 N-terminal amino acids and the other, in which 73 residues at the N terminus were replaced with the analogous sequence from ARL1, were not inhibited by arfaptin, consistent with the conclusion that arfaptin interaction requires the N terminus of ARF. This region has also been implicated in phospholipase D activation, but whether the two proteins interact with the same structural elements in ARF remains to be determined. Arfaptin inhibition of the action of ARF5 and ARF6 was less than that of ARF1 and ARF3; its effects were less on nonmyristoylated than myristoylated ARFs. Arfaptin effects on guanine nucleotide binding by ARFs were minimal whether or not a purified ARF guanine nucleotide-exchange protein was present. These findings indicate that arfaptin acts as an inhibitor of ARF actions in vitro, raising the possibility that it has a similar role in vivo.