above, the end result—the altered force distribution at the protein–lipid interface and en-
suing activation of the mechanosensitive protein—is the same in both models (Kung
2005).
According to another model, tether-mediated activation of membrane proteins occurs
when stress-deformed cytoskeletal/ECM components pull on the protein and thereby di-
rectly change the protein conformation [e.g., by shifting an autoinhibitory domain which
releases block and triggers activation (trapdoor model) (Hamill and Martinac 2001; Kung
2005)] (Figure 5.3) or by partially unfolding the attached protein and thus revealing pre-
viously hidden catalytic/binding sites (Janmey and Weitz 2004; Vogel 2006). Several
mechanosensitive ion channels of vertebrate and invertebrate organisms are proposed to
96 PLANT TROPISMS
Figure 5.2. Forces exerted through the membrane can gate ion channels. A. The force distribution through-
out the depth of the membrane leads to a steep gradient in gating force on the interface between the channel
protein and the lipid head groups. Membrane tension can favor channel opening if the cross-sectional area of
the open state of a channel is larger than the closed (B), or if membrane tension leads to thinning of the bi-
layer and the conformational change leading to the open state of the channel reduces the profile of the mem-
brane-spanning, hydrophobic residues on the channel surface (C).
Redrawn from Kung (2005).