Cell polarisation is a key biological process crucial for the functioning of essentially all cells. Regulation of cell polarity is achieved through various processes determined by both internal and external factors. An example of the latter is that cell polarity can be disrupted or lost as a consequence of a variety of external stresses. When facing such stresses, cells adapt to unfavourable conditions by activating a range of molecular signalling pathways, collectively termed ‘stress response’. Despite the connections between external stress and cell polarity, whether stress-response signalling regulates cell polarisation and what the molecular basis for such regulation remains an open question. The fission yeast Schizosaccharomyces pombe presents an excellent biological platform to study the complexity of cell polarity regulation on a systematic level. This study is aimed at understanding the functional relationship between stress-response signalling and maintenance of cell polarity in this model organism. The findings presented in this thesis set the basis for establishing a functional link between the activation of the S.pombe stress-response pathway and the activity of the master regulator of cell polarity- the Rho GTPase Cdc42. Here, I describe experiments that identify an active involvement of the stress-response mitogen-activated kinase (MAPK) Sty1 in the dispersal of active Cdc42 from the sites of growth. This new role for Sty1 occurs independently from its involvement in transcription regulation and other previously identified signalling pathways involving Sty1. Furthermore, I also find that Sty1’s involvement in Cdc42 regulation has direct implications for fission yeast physiology as it is essential for the maintenance of cellular quiescence upon nitrogen starvation. This thesis also focuses on identifying the targets of Sty1 orchestrating the active Cdc42 disruption. Here, I describe a candidate-based approach, where I investigate the role of proteins from the Cdc42 regulatory network during Sty1 activation. Additionally, I present a global phospho-proteomics approach to identify novel targets of Sty1 and offer preliminary findings which might explain Sty1’s involvement in Cdc42 regulation.

In Schizosaccharomyces pombe, the MAPK pathway Sty1 is activated upon several stress situations, like osmotic and oxidative stress, stationary phase, UV radiation or heat shock. Since the modulation of gene expression is one of the main outputs of this response, we focused this Thesis work on the charactherization of the transcription regulation by the activation of the Sty1 pathway and through the transcription factors Atf1 and Pcr1. Moreover, we extend our field of interest investigating how stress–related chromatin remodelers are affecting the stress defence transcription of the cells. / En Schizosaccharomyces pombe, la vía de la MAPK Sty1 es activada ante diferentes situaciones de estrés, como son el estrés oxidativo u osmótico, fase estacionaria, radiación UV o choque de calor. Al ser la modulación de la expresión génica uno de los más importantes objetivos de esta respuesta, hemos focalizado el trabajo de esta Tesis doctoral en la caracterización de la regulación transcripcional mediada por la activación de la ruta de Sty1 y los factores de transcripción Atf1 y Pcr1. Además, hemos ampliado nuestra área de interés investigando el papel de remodeladores de cromatina relacionados con la respuesta a estrés y cómo a participan en la transcripción estrés-dependiente.

La vida aeróbica conlleva la formación de especies reactivas derivadas del oxígeno: el radical hidroxilo (OH·), el ión superóxido (O2·-) y el peróxido de hidrógeno (H2O2). En Schizosaccharomyces pombe, dos rutas controlan las respuestas antioxidantes en respuesta a estrés oxidativo por H2O2: la del factor de transcripción Pap1 y la de la MAP quinasa Sty1. En esta tesis doctoral, hemos determinado que la activación de Pap1 se da en respuesta a dosis moderadas, pero no severas, de H2O2. Hemos identificado a la peroxirredoxina Tpx1 como sensor y transmisor de la señal de estrés oxidativo a Pap1. La inactivación temporal de Tpx1, durante estrés oxidativo severo, por oxidación a sulfínico de su cisteína catalítica inhibe la transmisión de señal a Pap1. En dichas condiciones, se activa la ruta de Sty1, que media la inducción de Srx1, cuya función es reducir y, con ello, reactivar a Tpx1. Finalmente, hemos estudiado el papel esencial de Tpx1 en aerobiosis. / Aerobic life involves formation of reactive oxygen species: hydroxyl radical (OH·), superoxide ion (O2·-) and hydrogen peroxide (H2O2). In Schizosaccharomyces pombe, two pathways respond to H2O2 and trigger independent antioxidant-gene responses: the Pap1 and the Sty1 pathways. In this thesis project, we have determined that the activation of the transcription factor Pap1 occurs only at low, but not elevated, H2O2 concentrations. We have identified the peroxiredoxin Tpx1 as a H2O2-sensor and redox activator of Pap1. The temporal inactivation of Tpx1 during severe oxidative stress, by oxidation of its catalytic cysteine to sulfinic acid, inhibits signal transduction to Pap1. During these conditions, the MAP kinase Sty1 is activated and expression of the sulfiredoxin Srx1 is triggered. Srx1 functions to reduce and thus reactivate Tpx1. Finally, we have analysed the essential function of Tpx1 in aerobiosis.