Mechanisms of MTOC clustering and spindle positioning in the budding yeast Cryptococcus neoformans: a microtubule grow-and-catch model

Mitotic spindle formation in the pathogenic budding yeast, Cryptococcus neoformans, depends on multitudes of inter-dependent interactions involving kinetochores (KTs), microtubules (MTs), spindle pole bodies (SPBs), and molecular motors. Before the formation of the mitotic spindle, multiple visible microtubule organizing centers (MTOCs), coalesce into a single focus to serve as an SPB. We propose a 9grow-and-catch9 model, in which cytoplasmic MTs (cMTs) nucleated by MTOCs grow and catch each other to promote MTOC clustering. Our quantitative modeling identifies multiple redundant mechanisms mediated by a combination of cMT-cell cortex interactions and inter-cMT coupling to facilitate MTOC clustering within the physiological time limit as determined by time-lapse live-cell microscopy. Besides, we screened various possible mechanisms by computational modeling and propose optimal conditions that favor proper spindle positioning - a critical determinant for timely chromosome segregation. These analyses also reveal that a combined effect of MT buckling, dynein pull, and cortical push maintain spatiotemporal spindle localization.