Num1 relieves dynein’s inhibitions

Lammers and Markus describe how dynein is activated at the budding yeast cell cortex so that it can pull on astral microtubules and position the mitotic spindle.

During mitosis, dynein motors are anchored at the yeast cell cortex by the receptor protein Num1. Dynein is transferred to Num1 from the plus ends of dynamic astral microtubules. Pac1 (a homologue of human LIS1) links dynein to the plus-end tracking protein Bik1 and prevents the motor from prematurely walking away toward the microtubule minus end. Once anchored to Num1, though, dynein’s minus end–directed motility is activated and the mitotic spindle is pulled into the bud. Whether Num1 directly activates dynein is unknown, however.

Lammers and Markus found that overexpressing the dynein-binding domain of Num1 prompted the motor protein’s premature disappearance from astral microtubule plus ends. Live imaging of cells expressing this Num1 domain showed dynein moving toward microtubule minus ends attached to the yeast spindle pole body, a phenomenon never seen in wild-type cells. The Num1 fragment reduced dynein’s colocalization with Pac1, suggesting that receptor binding might activate dynein by disrupting the motor’s association with its inhibitor. Accordingly, overexpressing Pac1, or a dynein mutant with increased affinity for Pac1, restored the motor protein’s localization to microtubule plus ends in cells expressing the Num1 fragment.

Senior author Steven Markus now wants to test his model of cortical dynein activation by reconstituting the process in vitro.