CALCIUM-DEPENDENT INHIBITION OF IN-VITRO THIN-FILAMENT MOTILITY BY NATIVE TITIN

Titin (also known as connectin) is a giant filamentous protein that sp ans the distance between the Z- and NI-lines of the vertebrate muscle sarcomere and plays a fundamental role in the generation of passive te nsion. Titin has been shown to bind strongly to myosin, making it tigh tly associated to the thick filament in the sarcomere. Recent observat ions have suggested the possibility that titin also interacts with act in, implying further functions of titin in muscle contraction, We show - using in vitro motility and binding assays - that native titin inte racts with both filamentous actin and reconstituted thin filaments. Th e interaction results in the inhibition of the filaments' in vitro mot ility. Furthermore, the thin-thin filament interaction occurs in a cal cium-dependent manner: increased calcium results in enhanced binding o f thin filaments to titin and greater suppression of in vitro motility .