ing. Thus, the cytoskeleton may not only act to convey forces to interior regions of the
cell by tugging on an attached mechanosensor, but may itself translate stress information
into biochemical reactions via altered binding to associated proteins (Figure 5.3d). For
example, specific cytoplasmic proteins show differential binding to the cytoskeleton de-
pendent on the state of cytoskeletal tension (Sawada and Sheetz 2002; Tamada et al.
2004). Similarly, stretching of the cytoskeleton promotes activation of a tyrosine kinase
(Scr) via interaction with an actin-binding protein (AFAP) (Han et al. 2004). Strain-
induced alterations in binding affinity of cytoskeletal proteins are thought to be mediated
via conformational changes, especially the unfolding of protein domains and resulting
exposure of cryptic binding (or catalytic) sites.
Indeed, mechanically induced partial unravelling of proteins is a potentially wide-
spread signalling mechanism. By using force spectroscopy to investigate the mechanical
properties of multimodular proteins, such as the giant muscle protein titin and the ECM
adhesion protein fibronectin, it was shown that, when exposed to increasing levels of ten-
sion, the modules unfolded one-by-one in a sequence determined by the mechanical sta-
bility of each domain (Zhuang and Rief 2003; Vogel 2006) (see Figure 5.3).The stability
should be dictated by features such as disulfide and hydrogen bonds that determine the
secondary structure of the protein. These features will in turn be modulated by cellular
environment such as pH and ionic strength and local redox potential (Vogel 2006). Thus,
the magnitude of stress experienced by the protein may well be encoded in the degree of
protein unfolding and signalled to the cell by way of revealing different recognition
(binding or catalytic) sites in each unravelled module (Vogel 2006). This is also an in-
triguing possibility for plant cells. If deformation of cell wall components results in ex-
posure of new binding sites, interactions with plasma membrane receptors may be newly
formed or broken to trigger signalling to the cell interior.
In addition to such direct effects on protein conformation, two other themes that
emerge from this overview of mechanosensory channel function are that gating through
tension in the lipid bilayer and through tethering to either the ECM and/or the cytoskele-
ton are the most prominent modes regulating the opening and closing of mechanosensory
channels. We will therefore describe in more detail two of the most intensively studied
channel types, the MscL channels ofEscherichia Coli and the MEC channels ofCaenor-
habditis elegans, to explore the molecular mechanisms that underlie each of these modes
of channel regulation. These channels may well provide clues to the structure of the elu-
sive plant mechanosensory complex.
5.3.1 Gating through Membrane Tension: The Mechanoreceptor for Hypo-osmotic
Stress in Bacteria, MscL
Free-living bacteria are exposed to very sudden changes in the osmolarity of their envi-
ronment. When a sudden drop in external osmolarity results in rapid influx of water and
threatens to cause lytic rupture of the cell, mechanically gated channels act as emergency
valves. Two of these channels, named mechanosensitive channels of small (MscS) and
large (MscL) conductance, have been characterized to be activated by increasing mem-
brane tension and enable the cell to rapidly jettison large amounts of osmolytes (Perozo
and Rees 2003).