A titinic extension

In a homage to reductionism, Hongbin Li, Julio Fernandez (Columbia University, New York, NY), and colleagues show that the properties of individual domains of titin, a giant muscle protein, can explain the elasticity of intact muscle.

The pulling in muscle is done by actin and myosin, but stretching is resisted by the elasticity of titin. Individual titin molecules of up to 3 MDa span over an entire half sarcomere—the unit of contraction in muscle. But only one region of titin confers elasticity, and this region can be broken down into discrete domains.

Fernandez and colleagues stretch various combinations of these domains by single molecule atomic force microscopy. They find that, under increasing force, proximal Ig domains undergo little passive stretching before giving way to a wholesale unfolding. The result is...