Correspondence to Miriam B. Goodman: m b g o o d m a n @ s t a n f o r d . e d uThe online version of this article contains supplemental material.
I N T R O D U C T I O NSensing force is ubiquitous in cellular life, with uses ranging from protection from osmotic shock in bacteria ( Levina et al., 1999 ) to the appreciation of the softest touch on human skin ( Johansson and Vallbo, 1979 ). Among medically relevant modes of force transduction in humans are fl ow regulation in the kidney, blood pressure regulation in vasculature, hearing, proprioception in musculature, nociception, and cutaneous touch. Each of these processes is likely to involve many molecules that have proven diffi cult to identify, possibly due to their rarity. In contrast, genetic analysis of touch in Caenorhabditis elegans has proven fruitful for identifying the molecular components of mechanotransduction machinery, including four membrane proteins (MEC-2, MEC-4, MEC-6, and MEC-10). These proteins form amiloride-sensitive, Na + -selective sensory mechanotransduction channels in the touch receptor neurons (TRNs) responsible for detecting low-intensity touches in C. elegans ( O ' Hagan et al., 2005 ). In Xenopus oocytes, these proteins also form amiloride-sensitive Na + channels ( Chelur et al., 2002 ;Goodman et al., 2002a ). MEC-4 and MEC-10 are pore-forming subunits that belong to the superfamily of degenerin/epithelial sodium channel (DEG/ENaC) ion channel proteins that are conserved in metazoans. DEG/ENaC proteins are preAbbreviations used in this paper: DEG, degenerin; ENaC, epithelial sodium channel; HDL, high-density lipoprotein; PHB, prohibitin homology; PON, paraoxonase; TRN, touch receptor neuron. dicted to share a common topology consisting of two transmembrane helices, intracellular N and C termini, and a large extracellular domain ( Kellenberger and Schild, 2002 ). Recent structural data confi rm this topology ( Jasti et al., 2007 ). MEC-2 and MEC-6 are auxiliary subunits required for channel function in vivo ( O ' Hagan et al., 2005 ). Replacing the wild-type alanine at the degenerin ( d) position in MEC-4 (A713) or MEC-10 (A673) with residues larger than cysteine induces TRN degeneration in vivo ( Driscoll and Chalfi e, 1991 ;Huang and Chalfi e, 1994 ) and increases channel open probability in Xenopus oocytes ( Brown et al., 2007 ). The d position is at the extracellular end of the second transmembrane domain and its side chain interacts with the fi rst transmembrane domain of the adjacent subunit ( Jasti et al., 2007 ). This structural observation is consistent with our fi nding that mutations at the d position in MEC-10 can both enhance and suppress gain-of-function mutations in MEC-4 ( Brown et al., 2007 ). Taking advantage of the increased open probability in mutants, we study gain-of-function mutants of both MEC-4 and MEC-10 with an alanine to threonine substitution at the d position. For clarity, we refer to these mutants as MEC-4d and MEC-10d and to heteromeric channels as MEC-4d/10d channels. The ion channel formed by the...