Kinesin-2 motors: Kinetics and biophysics

Abstract

Kinesin-2s are major transporters of cellular cargoes. This subfamily contains both homodimeric kinesins whose catalytic domains result from the same gene product and heterodimeric kinesins with motor domains derived from two different gene products. In this review, we focus on the progress to define the biochemical and biophysical properties of the kinesin-2 family members. Our understanding of their mechanochemical capabilities has been advanced by the ability to identify the kinesin-2 genes in multiple species, expression and purification of these motors for single molecule and ensemble assays, and development of new technologies enabling quantitative measurements of kinesin activity with greater sensitivity.Kinesins constitute a superfamily of microtubule-based molecular motor enzymes that couple the chemical energy from ATP turnover to force production for diverse cellular functions (reviewed in (1-12)). Kinesins are classified into 15 different subfamilies, yet they share a structurally conserved kinesin motor domain (1,3,(13)(14)(15)(16). However, key amino acid residue changes can confer unique mechanochemical properties to each kinesin, which in turn specify cellular function. The N-terminal kinesins are composed of an N-terminal motor domain connected to a long a-helical region that dimerizes into a coiled-coil stalk that ends with a C-terminal domain that may interact with specific adaptor proteins for cargo linkage (Fig. 1). N-kinesin subfamilies include conventional kinesin-1, kinesin-2, kinesin-3, kinesin-5 Eg5/KSP, and kinesin-7 CENP-E, and all are best known for their roles in intracellular transport.N-kinesins carry cargo directionally toward the plus-end of microtubules, which are polymerized from ab-tubulin subunits to form a cylindrical polymer of 13 protofilaments. The kinesins are able to "read" the polarity of the microtubule because of the structural asymmetry of ab-tubulin subunits. The movement of N-kinesins is designated as "processive," which implies that upon microtubule collision, a single, dimeric kinesin steps continuously toward the microtubule plus end in an asymmetric hand-over-hand manner hydrolyzing one ATP per 8-nm step for hundreds of steps (17)(18)(19)(20)(21)(22). The 8-nm step size results from the distance between adjacent ab-tubulin dimers along the microtubule lattice. As Fig. 2 illustrates, a processive kinesin binds the microtubule and then goes through a series of structural transitions, each modulated by nucleotide state. To maintain a processive run with continuous stepping, the Kinesin-2 Molecular Motors 2ATPase cycle of each head remains out-of-phase with the other to avoid premature release if both heads exist in a microtubule weak binding state simultaneously. The degree of processivity, quantified by "run length," varies between kinesin subfamilies and is regulated by a series of "gating" mechanisms in which a chemical and/or mechanical requirement must be satisfied to proceed forward. There has been significant effort to define the determinants of ...

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