1. Department of Biological Sciences, Columbia University, New York 10027, USA
2. These authors contributed equally to this work
3. Present address: Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, California 94305-5345, USA.

Correspondence to: Martin Chalfie¹ Correspondence and requests for materials should be addressed to M.C. (e-mail: Email: mc21@columbia.edu).

Touch sensitivity in animals relies on nerve endings in the skin that convert mechanical force into electrical signals. In the nematode Caenorhabditis elegans, gentle touch to the body wall is sensed by six mechanosensory neurons¹ that express two amiloride-sensitive Na⁺ channel proteins (DEG/ENaC). These proteins, MEC-4 and MEC-10, are required for touch sensation and can mutate to cause neuronal degeneration^{2, 3}. Here we show that these mutant or 'd' forms of MEC-4 and MEC-10 produce a constitutively active, amiloride-sensitive ionic current when co-expressed in Xenopus oocytes, but not on their own. MEC-2, a stomatin-related protein needed for touch sensitivity⁴, increased the activity of mutant channels about 40-fold and allowed currents to be detected with wild-type MEC-4 and MEC-10. Whereas neither the central, stomatin-like domain of MEC-2 nor human stomatin retained the activity of full-length MEC-2, both produced amiloride-sensitive currents with MEC-4d. Our findings indicate that MEC-2 regulates MEC-4/MEC-10 ion channels and raise the possibility that similar ion channels may be formed by stomatin-like proteins and DEG/ENaC proteins that are co-expressed in both vertebrates and invertebrates^{5, 6, 7, 8}. Some of these channels may mediate mechanosensory responses.