Fig. 9.

Titin segmental elongations for passive (A) and active (B) stretching of sarcomeres. (A) For passive stretches, the proximal segment of titin (diamond symbols) for both myofibrils shown here elongates with sarcomere elongation up to a sarcomere length of 4.0 µm, as shown for myofibril 1. (B) For active stretches, the proximal segment of titin (diamond symbols) elongates for part of sarcomere stretching but then stops elongating and remains at a constant length for the remainder of the sarcomere stretch. We interpret these data as shown in the schematic figures to the right of the raw data figures; that is, titin somehow binds to actin during active sarcomere or muscle stretching, and its proximal segments cannot elongate any farther because they are fixed on the “rigid” backbone of actin. PEVK = region of titin rich in proline, glutamate, valine, and lysine. M-line indicates the middle of the sarcomere and Z-line indicates the end of the sarcomere.