1. Department of Physiology and Biophysics, Mayo Foundation, Rochester, Minnesota 55905, USA
2. Institute of Physiology and Pathophysiology, University of Heidelberg, D-69120 Heidelberg, Germany
3. Donald Danforth Plant Science Center, St Louis, Missouri 63132, USA

Correspondence to: Julio M. Fernandez¹ Correspondence and requests for materials should be addressed to J.M.F. (e-mail: Email: jfernandez@columbia.edu).

Abstract

Through the study of single molecules it has become possible to explain the function of many of the complex molecular assemblies found in cells^{1, 2, 3, 4, 5}. The protein titin provides muscle with its passive elasticity. Each titin molecule extends over half a sarcomere, and its extensibility has been studied both in situ^{6, 7, 8, 9, 10} and at the level of single molecules^{11, 12, 13, 14}. These studies suggested that titin is not a simple entropic spring but has a complex structure-dependent elasticity. Here we use protein engineering and single-molecule atomic force microscopy¹⁵ to examine the mechanical components that form the elastic region of human cardiac titin^{16, 17}. We show that when these mechanical elements are combined, they explain the macroscopic behaviour of titin in intact muscle⁶. Our studies show the functional reconstitution of a protein from the sum of its parts.