Question

If muscle myosin is non-processive, then how is it that we can contract our muscles and hold them that way for a sustained period of time? That is, how can our muscles resist a counter force (for example a heavy object) and contract continuously when we lift it despite the fact that the motor protein, myosin II, is non-processive?

Answer 1

The myosin 2 family consists of skeletal,smooth cardiac,and non muscle myosin.Non musle myosin isoforms together with smooth musle myosin 2 ,form the group of smooth muscle and non muscle myosins.Myosin 2 molecular motors are characterized by their ability to form higher order oligomers called thick filaments These NMIIB thick filaments are bipolar and contain 14 dimers in each half .Non muscle myosin 2 is localised to stress fibres ans is involved in cell motility and cytokinesis moreover NMIIB is thought to be involved in tension generation in the cortex of neurons and in the tonic force generating phase of smooth muscle contraction.

The function of NMIIB requires it to be strongly attached to the actin to maintain tension.A myosin motor can accomplish this goal in several ways .it can have a high duty ratio meaning it is strongly bound to actin for the majority of the biochemical cycle or it can have force dependent kinetics which allow a myosin to change its duty ratio when experiencing a force.ADP release is thought to be force sensitive Transition of for myosin 2 motors.DDP release is slowed when myosin 2 experience a hindering force and accelerated when myosin 2 experience an assisting forve.Although the duty ratio of the NMIIB monomer is debated it is clear from biochemical experiments that the NMIIB dinner can bind to neighbouring actin monomers in a two headed confirmation.