Calcium-dependent inhibition of in vitro thin-filament motility by native titin

Titin (also known as connectin) is a giant filamentous protein that spans the distance between the Z- and M-lines of the vertebrate muscle sarcomere and plays a fundamental role in the generation of passive tension, Titin has been shown to bind strongly to myosin, making it tightly associated to the thick filament in the sarcomere. Recent observations have suggested the possibility that titin also interacts with actin, implying further functions of titin in muscle contraction. We show - using in vitro motility and binding assays - that native titin interacts with both filamentous actin and reconstituted thin filaments. The interaction results in the inhibition of the filaments' in vitro motility. Furthermore, the titin-thin filament interaction occurs in a calcium-dependent manner: increased calcium results in enhanced binding of thin filaments to titin and greater suppression of in vitro motility.