Development of a novel thiol reagent for probing ion channel structure: studies in a model system.

We have synthesized a novel thiol reagent, 2-[(methylsulfonyl)thio]ethyl [N-(N,N-dimethylamino)ethyl]carbamate (MTSAC), that contains a carbamate functional group as well as a (positively charged) terminal amino group. The carbamate C-N bond isomerizes on a millisecond time scale and significantly alters the three-dimensional shape of the reagent. The behavior of this reagent was contrasted with that of the commonly used thiol reagent, [(methylsulfonyl)thio]ethylamine MTSEA [Akabas, M. H., & Karlin, A. (1995) Biochemistry 34, 12496-12500], with respect to its effect on single-channel currents passing through modified gramicidin channels. While both reagents decreased single-channel currents, the MTSAC-treated channels also showed a pattern of steps in the current recordings on the time scale of the carbamate bond isomerization. Moreover, the pattern and size of these steps were sensitive to the location of the thiol-reactive site in relation to the channel entrance. Thus, MTSAC may prove useful as a reagent for establishing the proximity to the pore in studies of ion channel proteins of unknown structure.