Mitotic motors.

Abstract

T HE complex but well-controlled motions of chromosomes during mitosis have long evoked the view that the mitotic spindle contains enzymes capable oftransducing chemical energy into mechanical work, but the nature ofthese "motor" molecules has been elusive . During the last 40 years, three ideas have dominated the field : (a) microtubules (MT)' attach to chromosomes and move them by the addition and removal of tubulin; (b) mitotic forces are produced by mechanochemical enzymes, probably ATPases, that interact with MTs; and (c) mitotic forces are generated by muscle proteins. Possibility c now seems remote, but recent data from studies on the dynamics of spindle MTs, on the genes and gene products that are required for normal chromosome motion, and on the time-dependent changes in spindle structure reveal that both possibilities a and b are pertinent for explaining mitotic movements. While it is too soon to say just how chromosomes are moved during cell division, there is now enough information to identify key phenomena and the kinds of proteins involved . This paper is a sketch of the motile events that occur during mitosis and a discussion of the ways that MTs and their associated motor enzymes may cause these motions . Mitotic Motions and Forces MotionsThe essential features of chromosome motion are well known (Fig. 1). The interactions between MTs and chromosomes begin at the transition from prophase to prometaphase; in higher eukaryotes this occurs when the nuclear envelope disperses, while in many lower eukaryotes spindle proteins assemble in the nucleus . MTs grow out from the centrosomes (spindle poles) and interact with the chromosomes and with each other. Initial chromosome movements are usually directed toward the nearby pole and are often comparatively fast (ca. 25 pm/min). One ofthe two sister kinetochores is the principal site at which the force for this motion is generated (71). Eventually sister kinetochores attach to MTs growing from opposite poles, and each chromosome shows a net motion toward the spindle equator. During this "congression" to the metaphase plate, which is usually the sum of many movements toward and away from each pole, the rate ofchromosome motion decreases . Anaphase begins with the splitting of the centromeres, and sister chromatids move apart at 0.5-2 jAm/min . The decrease in distance be-1 . Abbreviations used in this paper : MT, microtubule .

Force Generation by Microtubule Assembly/Disassembly in Mitosis and Related Movements

“…Several motor proteins have been found, by immuno-chemical staining, to co-localize with specific regions of chromosomes, kinetochores, centrosomes, and spindle fiber microtubules or membrane vesicles (McIntosh and Pfarr, 1991;Goldstein, 1993;Sawin and Endow, 1993;reviewed in Fuller, 1995). In addition, genetic mutants of motor proteins, studied in particular in yeast and Drosophlia, have been shown to suppress or modify mitosis, indicating that they are important for spindle assembly, separation of the spindle poles, and chromosome movement (reviewed in: Goldstein, 1993;Sawin and Endow, 1993;Hoyt, 1994;Snyder, 1994;Fuller, 1995).…”

“…Since the late 1960's, following the discovery of dynein (Gibbons and Rowe, 1965), the focus on force generation for mitosis in most laboratories shifted to translocator motor proteins that generate sliding forces between or along microtubules (McIntosh et al, 1969;McIntosh and Koonce, 1989;McIntosh and Pfarr, 1991;Sawin and Endow, 1993 Song and Mandelkow, 1995). (B) Schematic summary of how microtubule assembly/disassembly is likely to be involved with chromosome movement and centrosome positioning in living cells.…”

Domains required for CENP-C assembly at the kinetochore.

“…Microtubules emanating from centrosomes dock at organelles on each sister chromatid known as kinetochores (Mitchison, 1988;McIntosh and Pfarr, 1991;Brinkley et al, 1992;Earnshaw and Tomkiel, 1992). After docking at the kinetochore, the alignment of chromosomes on a metaphase plate is achieved by the addition or loss of tubulin subunits at the ends of these attached microtubules and the simultaneous actions of microtubule motors within the kinetochore (Desai and Mitchison, 1995).…”

Motor activity and mitotic spindle localization of the Drosophila kinesin-like protein KLP61F.

“…These proteins are members of the kinesin superfamily of microtubule-associated motor proteins (Goldstein, 1993;Bloom and Endow, 1994;Hoyt, 1994), which use the energy of ATP hydrolysis to generate force and movement along microtubules (Vale et al, 1985). Together with various types of dynein, kinesin-like proteins may be responsible for the force production required during mitosis (McIntosh and Pfarr, 1991;Sawin and Scholey, 1991;Walker and Sheetz, 1993).…”