The TOXCAT assay system developed by Russ and Engelman [TOXCAT: a measure or transmembrane helix association in a biological membrane, Proc. Nail. Acad. Sci. USA 96 (1999) 863-868] provides an in vivo means of selecting for and evaluating the strength or interaction between identical transmembrane a-helices. In the course of Utilizing TOXCAT to study the architecture of a sodium Channel nel hNa(V)1.5, an apparently strong dimerization of two of its putative transmembrane segments was revealed. Following random mutagenesis of these regions, several amino acids critical for the observed dimerizations were identified. In order to develop a more efficient means of isolating mutations which specifically disrupt dimerization of these transmembrane segments without affecting their membrane-targeting properties, we developed a modification to the original TOXCAT design in which the C-terminal maltose binding protein moiety is replaced by the beta-lactamase. We show that this assay system is capable of simultaneously monitoring the integrity of the chimeric protein, its membrane insertion activity, and the ability of the transmembrane segment under study to dimerize. (c) 2005 Elsevier Inc. All rights reserved.