Identifying the interaction between Tea and Scd cell-polarity modules in S. pombe

Abstract

Cell polarity is essential for many cellular processes such as morphogenesis, migration, vesicle trafficking and asymmetric cell division. In Schizosaccharomyces pombe, cell polarity is regulated by two key modules. The Scd module drives polarized growth by regulating actin cytoskeleton, cell wall remodelling and exocytosis. The Tea module defines the location of the growth zone in a microtubule-dependent manner. While each module has been studied individually, the mechanisms by which they communicate remain unclear. Previous work in the Sawin Lab suggests that Scd1 associates with Tea proteins under non-stress conditions and that Tea1 influences Scd module recovery after stress. Therefore, this study aims to identify which proteins of the Scd and Tea modules interact and how these interactions regulate cell polarity, particularly under stress conditions.

To identify how the Tea module communicates with the Scd module, I took two complementary approaches. First, I used zero-length crosslinking coupled with mass spectrometry (MS) to identify novel candidate interactors between Tea and Scd module proteins. Second, I used an automated and quantitative fluorescence imaging assay to monitor Scd module behaviour during and after stress. To achieve this, I used Stress-Independent Sty1 Activation (SISA4) cells to precisely activate the stress-activated protein kinase (SAPK) pathway by removal of a small-molecule inhibitor. Upon SAPK activation, the Scd module disperses from cell tips to cell sides.

Upon SAPK inhibition, the Scd module recovers to cell tips. I found that deletion of Tea module proteins, particularly Tea1, results in delayed Scd module recovery.

Furthermore, upon SAPK activation, the Scd module signal disappears in cells lacking Tea1 and/or Tea4. These findings suggests that the Tea module is required for Scd module maintenance at cell tips during stress, rather than for recovery after stress. In particular, the analysis of Scd module behaviour in tea4Δ, dis2Δ, and tea4Δdis2Δ mutants supports a model in which Tea4 stabilizes Scd module signal at cell tips during SAPK activation, while phosphatase Dis2 antagonizes this function. Collectively, Tea module could be functioning as a cortical scaffold that is potentially anchoring polarity-associated proteins for efficient Scd module maintenance.

This maintenance may be fundamental for preserving cell polarity under fluctuating environmental conditions.