Counting spores

During meiosis, yeast cells use their spindle pole bodies (SPBs), their equivalent of centrosomes, as the basis for building a structure called the meiotic plaque. The plaque in turn forms the basis for forming a spore. On page 627, Taxis et al. present a model in which nutrition availability determines how many meiotic plaques and thus spores are formed.Three major protein components comprise the meiotic plaque, which is the site of assembly of spore membranes. If one of the three proteins is missing, plaques do not form and spores are not produced.

By monitoring the formation of meiotic plaques under different conditions, the team found a correlation between the amount of plaque constituents synthesized, the number of plaques generated, and nutrient abundance. Once proteins joined the plaque structure, they seemed to be locked in place, as happens in a crystal structure. No diffusion of the proteins back into the surrounding cytoplasm was detected. Thus, as plaque crystals formed, they could automatically limit the formation of plaques on the other SPBs by depleting the cell of available subunits.

Initiation of a plaque crystal was stochastic, though younger SPBs appeared to be more efficient at the process than were older ones. Mathematical modeling predicted that once a crystal starts to form, it creates more binding sites for additional subunits, and thereby increases the speed with which it grabs proteins out of solution. Thus, once a plaque started to form, it was more likely to complete its building project before another one could start. That means that when protein subunits are limiting, only some of the SPBs can generate meiotic plaques, and therefore the cell produces fewer spores.

It is not yet clear why older SPBs provide less favorable conditions for plaque formation, but the researchers have some hints. They speculate that it has to do with an accumulation of posttranslational modifications to proteins in the structure and are looking for mutants to bear out that hypothesis.