THE KAR3P AND KIP2P MOTORS FUNCTION ANTAGONISTICALLY AT THE SPINDLE POLES TO INFLUENCE CYTOPLASMIC MICROTUBULE NUMBERS

Microtubules provide the substrate for intracellular trafficking by as sociation with molecular motors of the kinesin and dynein superfamilie s, Motor proteins are generally thought to function as force generatin g units for transport of various cargoes along the microtubule polymer , Recent work suggests additional roles for motor proteins in changing the structure of the microtubule network itself, We report here that in the budding yeast Saccharomyces cerevisiae microtubule motors have antagonistic effects on microtubule numbers and lengths, As shown prev iously, loss of the Kar3p motor stimulates cytoplasmic microtubule gro wth while loss of Kip2p leads to a sharp reduction in cytoplasmic micr otubule numbers, Loss of both the Kip2p and Kar3p motors together in t he same cell produces an intermediate phenotype, suggesting that these two motors act in opposition to control cytoplasmic microtubule densi ty, A Kip2p-GFP fusion from single gene expression is most concentrate d at the spindle poles, as shown previously for an epitope tagged Kar3 p-HA, suggesting both of these motors act from the minus ends of the m icrotubules to influence microtubule numbers.