MTSEA [(2-aminoethyl) methanethiosulfonate] is a cysteine modifying agent [1], cysteine's sulfhydral side chain (-SH) can form a covalent disulfide bond with sulfur in the MTSEA reagent [2].
MTSEA was used to demonstrate the presence of a water-accessible cysteine within the binding-site crevice of the human dopamine D2 receptor [3]. MTSEA was demonstrated rotation of TM6 (upon receptor activation) by the appearance of an MTSEA-accessible cysteine in a constitutively active mutant of the β2-adrenergic receptor not accessible in the wild-type receptor [4]. The effect of MTSEA on ligand binding, in conjunction with site-directed mutagenesis, was used to define intramolecular contacts in the neurokinin-2 tachykinin receptor [5].
References:
[1]. Choi YB, Tenneti L, Le DA, Ortiz J, Bai G, Chen HS, Lipton SA. Molecular basis of NMDA receptor-coupled ion channel modulation by S-nitrosylation. Nature neuroscience. 2000 Jan;3(1):15-21.
[2]. O'Reilly JP, Shockett PE. Time-and state-dependent effects of methanethiosulfonate ethylammonium (MTSEA) exposure differ between heart and skeletal muscle voltage-gated Na+ channels. Biochimica et Biophysica Acta (BBA)-Biomembranes. 2012 Mar 1;1818(3):443-7.
[3]. Javitch JA, Li X, Kaback J, Karlin A. A cysteine residue in the third membrane-spanning segment of the human D2 dopamine receptor is exposed in the binding-site crevice. Proceedings of the National Academy of Sciences. 1994 Oct 25;91(22):10355-9.
[4]. Rasmussen SG, Jensen AD, Liapakis G, Ghanouni P, Javitch JA, Gether U. Mutation of a highly conserved aspartic acid in the β2 adrenergic receptor: constitutive activation, structural instability, and conformational rearrangement of transmembrane segment 6. Molecular pharmacology. 1999 Jul 1;56(1):175-84.
[5]. Bhogal N, Blaney FE, Ingley PM, Rees J, Findlay JB. Evidence for the proximity of the extreme N-terminus of the neurokinin-2 (NK2) tachykinin receptor to cys167 in the putative fourth transmembrane helix. Biochemistry. 2004 Mar 23;43(11):3027-38.