THE INTERPLAY BETWEEN THE ENDOPLASMIC RETICULUM STRUCTURE AND THE CYTOSKELETON ORGANIZATION IN SCHIZOSACCHAROMYCES POMBE

Abstract

In the rod-shaped fission yeast Schizosaccharomyces pombe (S. pombe), the ER consists of the sheet-like nuclear envelope (NE) and the cortical ER that forms an intricate network tightly apposing to the plasma membrane (PM). In this work, I show that the type V myosins and actin cables effectively transport the cortical ER into the growing S. pombe cell tips. Moreover, the ER is tethered to the lateral cell cortex by the highly conserved vesicle-associated membrane protein-associated (VAP) proteins Scs2 and Scs22. I further demonstrate that the cortical ER network is maintained by a set of three membrane proteins: reticulon/Rtn1, DP1/Yop1 and a newly identified evolutionarily conserved protein Tts1. In the absence of the ER tubulating proteins, the ER network structure is lost. As a result, the large ER cisternae physically shield the PM preventing the recruitment of the key division site regulator Mid1 and actomyosin ring assembly at the equatorial cortex. Strikingly, the detachment of the ER from the PM alleviates the division site positioning defects in cells with impaired ER architecture. We thus propose that in cells with prominent ER-PM contacts, fine reticulation of the ER network allows to establish sufficient well-distributed plasma membrane surfaces accessible for binding of peripheral protein complexes.