Cell polarity and sterol-rich membrane domains in fission yeast

Abstract

Fission yeast (Schizosaccharomyces pombe) is a model organism widely used for studying cell polarization. Polarization in fission yeast involves cytoskeleton-mediated positioning of growth sites. A complex consisting of the polarity factors Tea1 and Tea4 is transported on microtubules to the cell end regions. Tea4 interacts with the actin polymerization promoter For3. Selective activation of For3 in the cell end regions makes them rich in F-actin that contributes to localized cell growth. In this study, sterol-rich membrane (SRM) domains present at the growth sites are introduced as a new element in this picture, and their role in cell polarity establishment is analyzed. Since SRMs are absent from the plasma membrane in starved cells, imaging of cells recovering from starvation using the novel SRM marker GFP-Tna1 was performed to follow SRM domain formation de novo. Automated image analysis software was developed to analyze and correlate cell growth and SRM dynamics with an unprecedented level of precision. The results show that properly formed SRM domains are essential for fission yeast growth. SRMs, and with them the growth machinery, have to polarize before cell growth initiation. F-actin is required for selective removal of SRM domains in the cell middle region, and thus for polarizing SRMs. Fast removal of SRM domains in the cell middle region is not due to increased endocytic activity (mediated by F-actin). Tea1 controls the localization of polarized growth via Tea4 by affecting the positioning of SRM domains. Tea1 and Tea4 are essential for the stability of the SRM domains not associated with active growth sites. The importance of the microtubule cytoskeleton for the stability of SRM positioning stems from its role in the transport of the Tea1-Tea4 complex to the cell end regions. Tea1∆ and tea4∆ cells, known to grow monopolarly, grow faster at individual cell ends than wild type cells. Tea1 and Tea4 are required for proper timing of growth initiation. The proteins associated with the actin cytoskeleton, For3 and its activator Bud6, are important for the stability of SRM domains at both cell ends. For3 is also important for growth speed stabilization. In conclusion, a complex feedback loop links SRMs and cell growth. SRMs are essential for the polarization of the growth machinery, probably serving as platforms for its recruitment. The growth machinery, in turn, seems to stabilize SRM domains at sites that have initiated growth. Importantly, the results of this study show that SRMs are a critical factor in de novo cell polarization, and not merely a player in its maintenance, as was previously thought.