Multiple conformations of PEVK proteins detected by single-molecule techniques

An important component of muscle elasticity is the PEVK region of titin, so named because of the preponderance of these amino acids. However, the PEVK region, similar to other elastomeric proteins, is thought to form a random coil and therefore its structure cannot be determined by standard techniqu es. Here we combine single-molecule electron microscopy and atomic force mi croscopy to examine the conformations of the human cardiac titin PEVK regio n. In contrast to a simple random coil, we have found that cardiac PEVK sho ws a wide range of elastic conformations with end-to-end distances ranging from 9 to 24 nm and persistence lengths from 0.4 to 2.5 nm. Individual PEVK molecules retained their distinctive elastic conformations through many st retch-relaxation cycles, consistent with the view that these PEVK conformer s cannot be interconverted by force. The multiple elastic conformations of cardiac PEVK may result from varying degrees of proline isomerization. The single-molecule techniques demonstrated here may help elucidate the conform ation of other proteins that lack a well-defined structure.