Tugging at Heartstrings

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Science  07 Aug 2009:
Vol. 325, Issue 5941, pp. 657
DOI: 10.1126/science.325_657a
CREDIT: BERTZ ET AL., PROC. NATL. ACAD. SCI. U.S.A. 106, 10.1073/PNAS.0902312106 (2009)

One of the first lessons in learning to work with wood is that it exhibits a resistance to compression or tension that depends upon the direction in which force is applied. A macroscopic clue to this anisotropic behavior can be found by examining the wood grain. Bertz et al. demonstrate that this phenomenon can also be observed at the molecular level in their atomic force microscopy investigation of titin, a structural protein found in skeletal and cardiac muscle. In these tissues titin is subject to considerable force, and the endcap protein, telethonin (red), grasps two titin molecules, binding to the N-terminal domain Z1 (dark green and blue) and the next interior one Z2 (light green and blue). Pulling apart this joint requires a force of 700 pN when applied to Z2 (that is, the normal physiological direction of pulling), but only one-third as much force need be applied to Z1; the difference is accounted for by directionally oriented hydrogen bonds (gray) between telethonin and the Z2 domain.

Proc. Natl. Acad. Sci. U.S.A. 106, 10.1073/pnas.0902312106 (2009).

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