Calmodulin light chains are essential for large 35 nm steps of higher plant myosin XI

Abstract

Higher plant myosin XI which can moves processively on an actin filament with 35nm steps at high velocity (7μm/sec) has two head domains followed by long necks. At each neck domain 6 light chains including calmodulins (CaMs) attached on. In vitro assay revealed that processivity of myosin XI reduced concomitant with reversal detachment of CaMs, when it was exposed to high calcium ion (Ca²⁺). Estimation of the elasticity of a single myosin molecule suggested that removal of CaMs makes myosin XI shorter. Single molecule measurement revealed that myosin XI at pCa 4 still moves processively, but the step size reduced to 27nm at high load (-0.5 pN) and 22nm at low load (0.5-1 pN). These results show that neck domain acquires the rigidity and length by attachment of CaMs and act as a lever to generate large 35 nm steps.