MTOC clustering and nuclear division through proper spindle positioning in yeast: a microtubule grow-and-catch model

Abstract

Mitotic spindle formation in pathogenic budding yeast, Cryptococcus neoformans, depends on multitudes of interdependent 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 ‘grow-and-catch’ model, in which cytoplasmic MTs (cMTs) nucleated by MTOCs grow and catch each other, promote MTOC clustering. Our quantitative modeling identifies multiple redundant mechanisms mediated by cMT-cell cortex interactions and inter-cMT coupling, facilitating MTOC clustering within the experimental time frame. Implementing a similar stochastic model in mitotic spindle where MTs ‘grow-and-catch’ KTs, we demonstrate that impaired error correction leads to an erroneous partitioning of chromosomes between mother and daughter cells with equal proportions. However, either a marginal delay in the MT nucleation from the SPB of daughter bud (dSPB) or an enhanced MT nucleation from the SPB in mother (mSPB) needs to account for asymmetric chromosome segregation, as observed in Ipl1 depleted cells. Using an analytical model screen, we predict plausible mechanistic conditions for proper spindle positioning. We find that MT buckling together with dynein pull and cortical push maintains proper spindle localization.