Cytokinesis is the final stage of the eukaryotic cell cycle where, after chromosome segregation, the formation of a cleavage furrow separates the cell into two new cells. Furrow formation is achieved by closure of a contractile actomyosin ring (CR) that invaginates the plasma membrane. In fungal cells, it also requires simultaneous synthesis of a division septum structure that will constitute the new cell wall upon cell separation. Thus, the septum biosynthesis must be strictly coordinated with the deposition of new plasma membrane material and CR closure. The cell wall is an external structure, essential for the maintenance of fungal cell integrity. The α(1,3) and β(1,3)glucans are the two main structural polymers of fission yeast cell wall. Schizosaccharomyces pombe contains four β(1,3)glucan synthase subunits, Bgs1 to Bgs4, that share high identity with fungal and plant homologues. Interestingly, all of them and the α(1,3)glucan synthase Ags1 are essential. Our group is interested in the study of Ags1, Bgs1 and Bgs4, the main subunits involved in the synthesis of the different cell wall and septum glucans. The septum is a three-layered structure of a middle primary septum (PS) flanked by a secondary septum (SS) on each side. Bgs1 is responsible for the linear β(1,3)glucan synthesis of the PS which interacts specifically and with high affinity with the fluorochrome Calcofluor white (CW). Bgs1 shares an essential function with CR paxillin, collaborating for septum formation. Bgs4 is responsible for the branched β(1,3)glucan which is required for connecting extracellular cell wall and CR during cytokinesis; and Ags1 for the α(1,3)glucan which is essential for the PS adhesion strength needed to withstand the turgor pressure during cell abscission. Both Bgs4 and Ags1 are responsible for the SS formation and both are essential for cell integrity. Our group is interested in studying the cytokinesis process. It is generally believed in all eukaryote cells that furrow ingression and CR constriction initiate after nuclear mitosis completion at the end of anaphase or the onset of telophase. Thus, it is assumed a sequential process in which furrow ingression will not start until mitosis completion, when the chromosomes are segregated and the spindle disassembled. However, we have found that septum synthesis and therefore CR constriction, initiate in early-anaphase overlapping nuclear mitosis. The regulation of septation onset with regards to the CR proteins, activation of the septation initiation network (SIN), cell cycle mitotic kinase and cyclin, cell size, spindle pole body asymmetry, etc. was studied. Our data show that mitosis and septation are overlapped processes, but still ensuring a safe mitosis in which septum synthesis only initiates after successful chromosome segregation.

Resumen del trabajo presentado a la 10ª Reunión de la Red Española de Levaduras, celebrada en El Escorial (Madrid) del 16 al 18 de diciembre de 2015.

Peer Reviewed