Z-Line Elongation Observed in Titin Labeled Myofibrils

Abstract

A muscle myofibril (below) is composed of sarcomeres in series. These sarcomeres are made of myosin, actin and titin, representing the three major filamentous proteins in muscle. Titin links myosin in the A-band region, to actin 100nm from the Z-line. This particular junction where titin binds actin, is where T12 antibody localizes. Using single myofibrils isolated from rabbit psoas muscle, we labeled titin with T12 conjugated to quantum dots and stretched myofibrils between a cantilever and rigid glass needle. Preliminary data suggests that myofibrils display a broadening between these T12 bands when stretched, despite being considered mainly inelastic due to titin-actin interactions. Interestingly, densitometric analysis revealed the distance between flanking T12 bands increased, while the width of the fluorescent bands remained unchanged. Two alternative explanations could require the width of the Z-line to increase with stretch (Tonino et al., 2009), or titin could dislodge from actin to accommodate stretch. Given the longest average sarcomere length was 3.07μm, the actin dislodging seems less likely as the psoas muscle has not approached peak tension (∼3.9μm (Wang et al., 1991)). Thus, it appears that Z-line width increases with stretch to a greater extent than previously reported.View Large Image | View Hi-Res Image | Download PowerPoint Slide