ABSTRACT
Biological motors are quite different in their structures, adapted to their cellular functions. In general, they are composed of a mechanical frame that consists of both moving and static parts (defined by their function during movement), along with an energy supply. The mechanical frame is formed by proteins in most molecular motors, except for the RNA hexamer as in the phi29 dsDNA packaging motor. In most cases, the energy supply of biomotors comes from a proton gradient, or more typically, ATP (binding or hydrolysis), which leads to entropic/conformational changes of the biomotor and results in its movement. All these motor proteins have multiple domains to exert catalyzed biochemical reactions to support their function. Most biomotors are track-laying proteins: cytoskeletal track for linear motors, and DNA or RNA for DNA translocases or RNA polymerases. In this chapter, we describe the detailed structure of revolving biomotors.
In revolving motors, their substrates revolve around the inner side of the channel during advancement. The primary structural characteristic of revolving DNA packaging motors that allows them to be visually distinguished from rotation motors is related to their chirality and channel size. The diameter of the channel in revolving motors is wider than that of rotation motors, not only because it is translocated by both strands of DNA but also to allow the right-handed dsDNA to translocate the left-handed channel of rotation motors. In contrast, the channel of rotating motors is right-handed, with close contact between the chiral single DNA strand and channel wall (see Chapter 5 for more details).
There are other structural peculiarities that are typically found in revolving motors. A common feature of members of the ASCE superfamily is the hexameric arrangement of their components. Prior to this discovery, it was widely believed that there was a fivefold/sixfold mismatch gearing mechanism in these motors. Numerous assays have dispelled this hypothesis. In this chapter, we discuss how the debate about the stoichiometry of ASCE family members was ended, and describe the detailed structure of DsDNA translocases from the FtsK/SpoIIIE superfamily as typical revolving motors.
