Autophagy is a conserved bulk protein degradation process that is proposed to play a role in events that arise when organisms are forced to radically change their fate, including nutritional starvation, differentiation and development. In our present study, we have identified fission yeast autophagy as a bulk protein degradation process induced by the deprivation of environmental nitrogen, the effects of which are known to trigger sexual differentiation as an adaptive response. Autophagy-defective mutants were found to be sterile in the absence of environmental nitrogen, but could complete sexual differentiation when nitrogen was supplied, suggesting that the major function of autophagy is to provide a nitrogen source. In addition, the environmental nitrogen levels act as an autophagy "on/off " switch, whereas components essential for sexual differentiation were dispensable for this regulation. We propose that fission yeast autophagy functions to supply nitrogen and is activated when cells cannot access exogenous nitrogen, thus ensuring that they can adapt and subsequently propagate.

Microtubules and the motor protein dynein play pivotal roles in the movement and positioning of the nucleus and cytoplasmic organelles in a cell. In fission yeast, oscillatory movement of the nucleus termed horsetail nuclear movement (HNM) has been observed during meiotic prophase. HNM is led by an astral microtubule array emanating from the spindle pole body (SPB), a centrosome-equivalent organelle in yeasts, aided by the dynein-dynactin complex, and is proposed to facilitate the alignment of homologous chromosomes necessary for efficient meiotic recombination. Here we show that a meiosis-specific SPB component Hrs1p (also known as Mcp6p) is a key molecule to remodel microtubules into the horsetail-astral array (HAA). Deletion of Hrs1p impaired HAA formation, leading to compromised HNM. Ectopic expression of Hrs1p during the mitotic cell cycle resulted in the formation of a HAA-like astral microtubule array, which drove an oscillatory nuclear movement in interphase cells. Hrs1p interacted with components of the gamma-tubulin ring complex (gamma-TuRC) as well as with a meiotic SPB component. We propose that Hrs1p facilitates formation of the HAA, responsible for the vigorous HNM, by stabilizing connection between the SPB and minus ends of microtubules.