ADP-ribosylation factors: a family of ~20-kDa guanine nucleotide-binding proteins that activate cholera toxin

Abstract

ADP-ribosylation factors (ARFs) comprise a family of ~20 kDa guanine nucleotide-binding proteins that were discovered as one of several cofactors required in cholera toxin-catalyzed ADP-ribosylation of G, the guanine nucleotide-binding protein responsible for stimulation of adenylyl cyclase, and was subsequently found to enhance all cholera toxin-catalyzed reactions and to directly interact with, and activate the toxin. ARF is dependent on GTP or its analogues for activity, binds GTP with high affinity in the presence of dimyristoylphosphatidylcholine/cholate and contains consensus sequences for GTP-binding and hydrolysis. Six mammalian family members have been identified which have been classified into three groups (Class I, II, and III) based on size, deduced amino acid sequence identity, phylogenetic analysis and gene structure. ARFs are ubiquitous among eukaryotes, with a deduced amino acid sequence that is highly conserved across diverse species. They have recently been shown to associate with phospholipid and Golgi membranes in a GTP-dependent manner and are involved in regulating vesicular transport. (Mol Cell Biochem 138: 157-166, 1994)

ADP‐ribosylation factor 6 acts as an allosteric activator for the folded but not disordered cholera toxin A1 polypeptide

“…Thus, point mutations in the Y149 residue were predicted to disrupt CTA1–ARF interactions without eliminating the basal enzymatic activity of the toxin. We chose this approach to study CTA1–ARF interactions because any ARF protein can activate CTA1 (Price et al ., ; Welsh et al ., ), and it is not practical to eliminate the in vivo expression or activity of all ARFs. The established role of ARF1 in CT trafficking to the ER (Morinaga et al ., ; Richards et al ., ) would further complicate studies of CTA1–ARF interactions in the cytosol, as the loss of ARF1 function would disrupt an event upstream of toxin activity in the cytosol.…”

“…Consistent with this model, we have reported that lipid rafts exhibit a chaperone‐like interaction with CTA1 which is required for optimal in vivo toxin activity (Ray et al ., ). Gsα is located in lipid rafts at the cytoplasmic face of the plasma membrane (Oh and Schnitzer, ; Allen et al ., ; Kamata et al ., ), and, in vivo , GTP‐bound ARF proteins associate with membranes by virtue of their N‐terminal lipid modifications (Welsh et al ., ; Moss and Vaughan, ). Thus, the host factors required for optimal CTA1 activity are in proximity to the Gsα target.…”

“…The raft‐induced gain of toxin function at 37°C thus appeared to result from a simple gain‐of‐structure which placed CTA1 in a folded conformation with basal activity. Optimal stimulation of CTA1 likely requires an allosteric interaction with host ADP‐ribosylation factors (ARFs) (Welsh et al ., ).…”

“…ARF proteins are small molecular weight GTPases that were originally isolated from cell extracts as in vitro allosteric activators of CTA1 (Kahn and Gilman, 1984; 1986; Tsai et al ., 1987; 1988). There are six members of the ARF family grouped in three classes, and each class can activate CTA1 (Price et al ., ; Welsh et al ., ; Moss and Vaughan, ). ARF6 can bind to CTA1 over a range of temperatures in either the absence or the presence of GTP (Pande et al ., ), but only ARF/GTP can stimulate the ADP‐ribosyltransferase activity of CTA1 (Kahn and Gilman, 1984; 1986; Tsai et al ., ; Bobak et al ., ; Price et al ., ).…”

TcArf1: a Trypanosoma cruzi ADP-ribosylation factor

“…In metazoans, three distinct classes of Arf proteins can be discriminated through phylogenetic analysis of peptide sequence and gene structure (Welsh et al 1994). However, this division is still obscure for plants, fungi and protists.…”