Binding of triton X-100 to diphtheria toxin, crossreacting material 45, and their fragments.

Abstract

Binding of the nonionic detergent [3H]Triton X-100 by diphtheria toxin, by the nontoxic serologically related protein crossreacting material (CRM) 45 and by their respective A and B fragments was studied. If first denatured in 0.1% sodium dodecyl sulfate, all of the proteins with the exception of fragment A bind increasing amounts of Triton X-100, reaching a maximum of > 40 mol bound/mol protein when the detergent concentration exceeds its critical micelle concentration. No measurable amount of Triton X-100 is bound by native toxin or its A fragment at any concentration of the detergent. Undenatured CRM45 or its B45 fragment readily become inserted into Triton X-100 micelles when the detergent reaches its critical micelle concentration. The toxin molecule apparently contains a hydrophobic domain located on the portion of the B fragment that is linked to A. This region is masked in native toxin. Based on these findings, a model is proposed to describe how fragment B facilitates the transport of the enzymically active hydrophilic fragment A across the plasma membrane [mammalian] to reach the cytoplasm.