Caracterización de la función de PP2A en la morfogénesis de "Schizosaccharomyces pombe"

Résumé

We are aimed at understanding how cells acquire their characteristic shape. To study these questions, we are using a simple eukaryote, genetically tractable fission yeast Schizosaccharomyces pombe, as a model organism. Fission yeast is a useful model for addressing these problems, due to its easy genetics, availability of genome-wide collection of gene deletions and well developed microscopic, genetic and molecular biology tools. S. pombe polarized growth is produced via hierarchical regulatory interactions of microtubule-dependent cortical polarity markers with regulators of actin polymerization and vesicle secretion. In addition, intracellular signalling pathways act to maintain the polarized state and to control cellular decisions in response to environmental conditions, as for instance, in growth response to pheromone gradients. The MOR pathway is an evolutionarily conserved signalling pathway that controls cellular morphogenesis and cell separation in yeasts and fungi. This pathway is composed of a module of three kinases, known in S. pombe as Ste20-like, Nak1 or Orb3 linked to the scafolding protein Mor2 that is associated with the plasma membrane. This module signals to the NDR Orb6 and its coactivator Mob2 that activate downstream targets to regulate cell polarity and gene expression. The role of the MOR pathway in the regulation of polarized growth is poorly understood. Some evidence suggests that the downstream NDR kinase regulates polarized growth by controlling Golgi- and Sec2/Sec4-dependent transport processes. In S. pombe the NDR-kinase Orb6 controls polarized cell growth by spatial regulation of Cdc42, a universal regulator of cell polarity. In the fission yeast, inactivation of the MOR pathway results in the impairment of cell polarization giving rise to round unpolarized cells. In this work we have investigated how fission yeast cells establish and maintain growth zones to restrict cell growth to the tips of the cylindrical cell. We have characterized a new role of PP2A and the MOR morphogenesis pathway in the spatial control of Cdc42 activity. This data suggest that this control occurs via regulation of non-growing cellular domain containing Rga4, a Cdc42 inhibitor, by PP2A in conjunction with the NDR kinase Orb6. Rga4 forms a nodular structure in the central lateral cortex of fission yeast cells and is excluded from the tips. Our data demonstrate that PP2A and the MOR signaling pathway regulate the Rga4 nodular structure as well as its dynamic properties. Inactivation of the MOR or PP2A results in the progressive loss of the Rga4 nodular domain, leading to the loss of cell polarity and the ectopic growth at this lateral domain. Also, using a proteome-wide screeng for PP2A substrates we have identified a number of proteins likely regulated by PP2A, some of which are, in fact, components of the MOR signalling pathway.