O’Hagan et al. (2005) were able to show that mechanical stimulation triggers a rapidly ac-
tivating mechanoreceptor current that is carried mostly by Na+and depolarizes the plasma
membrane. The membrane depolarization is then thought to elicit an increase in cytosolic
Ca2+by activating voltage-sensitive L-type Ca2+channels (Suzuki et al. 2003), highlight-
ing that a mechanosensory channel need not be directly Ca2+-permeable to elicit the touch-
induced Ca2+increases proposed to transduce the mechanical signal in plants (see below).
Saturating mutagenesis analysis has identified 18 genes required for this C. elegans
mechanosensitivity (named mechanosensory abnormal, or MEC proteins; Ernstrom and
Chalfie 2002), some of which encode transcription factors, cytoskeletal proteins, or pro-
teins of the ECM (Tavernarakis and Driscoll 1997). At least four of the MEC proteins
(MEC-2, -4, -6, and -10) form the channel complex producing the mechanoreceptor current
(O’Hagan et al. 2005) (Figure 5.4band Color Section). MEC-4 and MEC-10 are pore-
forming channel subunits and belong to the DEG/ENaC (degenerin/epithelial sodium chan-
nel) family of amiloride-sensitive Na+-conducting channels found exclusively in animals
(Lai et al. 1996; Goodman et al. 2002; Kellenberger and Schild 2002). MEC-6 is a single-
pass transmembrane protein with a cytoplasmic N-terminus and large extracellular C-
terminal domain (Chelur et al. 2002), whereas MEC-2, a monotopic protein with a stomatin-
like region, does not span the plasma membrane but is associated with the cytoplasmic side
of the lipid bilayer (Goodman et al. 2002). Both MEC-2 and MEC-6 interact with MEC-4
and MEC-10 to regulate channel conductance (Chelur et al. 2002; Goodman et al. 2002).
Interestingly, no mechanically induced activation of current was observed when the
wild-type channel complex was heterologously expressed in Xenopusoocytes (Goodman
et al. 2002). This observation suggests that mechanical gating is not accomplished by
changes in membrane tension alone, but requires additional force-transmitting elements
such as proteins of the ECM and the microtubule cytoskeleton physically tethered to the
channel complex. It has been proposed that the touch-induced movement of the two pu-
tative tethering sites (ECM-channel extracellularly and microtubule-channel intracellu-
larly) relative to each other provides gating tension and activates the channel (reviewed
by Tavernarakis and Driscoll 1997). Indeed, both the pore-forming channel subunits
MEC-4 and MEC-10 as well as MEC-6 have large extracellular regions thought to be im-
portant for interaction with the ECM proteins MEC-1, MEC-5, and MEC-9 (Chelur et al.
2002; Emtage et al. 2004).
The precise role of the prominent microtubule cytoskeleton in mechanosensation is
less clear. Mutations in the genes MEC-7andMEC-12cause loss of mechanoresponse.
These genes encode ß- and -tubulins, respectively, which are required to form micro-
tubule protofilaments. These microtubules are proposed to be tethered to the channel
complex via MEC-2 (Tavernarakis and Driscoll 1997) (Figure 5.4b). However, mutants
inMEC-7still show (attenuated) touch-triggered mechanoreceptor currents, suggesting
that direct linkage of the channel complex to microtubules is not an absolute requirement
for gating (O’Hagan et al. 2005).
5.3.3 Evidence for mechanically gated ion channels in plants
Although no plant mechanosensor has been unequivocally identified at the molecular
level, there is strong evidence for mechanically gated ion channels in the plasma mem-