Biological activity of preformed cholera toxin-ganglioside GM1 complex

Synthetic and natural amphiphiles, octyl glucoside, Nonidet P40, sodium dodecyl sulfate (SDS), gangliosides GM1 and GD1a, interact with cholera toxin (CLT) and with its active region (promoter A). The formation of CLT-amphiphile complex leads to inhibition of ADP-ribosyltransferase activity, a characteristic of promoter A elicited after thiol-reagents treatment. In all cases the interaction produces the maximum inhibitory effect above the critical micellar concentration of amphiphiles, although monomers of SDS show inhibition activity as well. The gangliosides appear to be capable of altering bilayer organization of membrane, similar to synthetic detergents. When CLT-ganglioside complexes were incubated with cell culture medium containing 10% fetal calf serum (FCS) and ADP-ribosyltransferase activity was completely restored both in cholera toxin and in promoter A. Some protein of FCS, which is avid of gangliosides, seems to be responsible for reversibility of inhibition. The results indicate that the active site of promoter A may be located in a hydrophobic pocket of the toxin structure. Furthermore, CLT was bound to reconstituted Sendai virus envelopes (RSVEs), containing a small amount of GM1. The RSVEs are made of membranous vesicles, capable of binding and fusing with host cell membrane. The incubation for 1 1hr of RSVE bearing CLT with Friend's erythroleukemic cells produced the stimulation of adenylate cyclase. This stimulation appears to be due to the translocation of the active subunit of CLT in the inner half of plasma membrane.