Our laboratory has been studying the biosynthesis and structure of the yeast cell wall and division septum as models for morphogenesis. beta-1,3-Glucan is the major structural component of the cell wall. In previous years, we found that the GTP-binding protein Rho1p is an essential subunit and regulator of beta-1,3-glucan synthase. Because Rho1p has also other essential functions in the cell, we obtained temperature sensitive rho1 mutants that appear to be specifically defective in the glucan synthesis function and we used the mutants to find out the consequences of arresting the formation of the polysaccharide. Electron microscopy showed that lysis occurred at the tip of very small buds, consistent with the need for beta-1,3-glucan for integrity of the cell wall. We also observed that, concomitant with the reduction in glucose incorporation in the mutants glucan, there was also a decrease in formation of the other major component of the cell wall, mannoprotein. This result suggests that beta-1,3-glucan is the scaffold or primer upon which the remainder of the cell wall is built. Work on this project is still in progress. Cytokinesis in yeast occurs by the simultaneous invagination of the plasma membrane, caused by contraction of an actomyosin ring, and by the deposition of chitin in the invagination, catalyzed by the enzyme chitin synthase 2. Continuation of this process leads to pinching off of mother and daughter cell plasma membranes and deposition of a disk, the chitin primary septum. This is followed by formation of secondary septa, whereby a trilaminar complete septum is generated. We found that mutants deficient in contractile ring (myo1) or in primary septum (chs2) formation show a remarkably similar phenotype. In both cases cytokinesis occurs, but in an abnormal fashion: there is no membrane invagination; cell walls progressively thicken at the mother-daughter cell neck, until the membranes coalesce and a thick septum, lacking the chitin disk, is formed. myo1 chs2 double mutants show the same defect as single mutants. Thus, it appears that contraction of the actomyosin ring and primary septum formation are two interdependent components of the septation process. We interpret the formation of abnormal septa found in the mutants as secondary septa growing at right angles with the normal orientation, because of the absence of the template furnished by the primary septum. For the formation of the salvage septa, another chitin synthase, Chs3p, seems to be necessary, because chs3 mutants are synthetically lethal with either chs2 or myo1 mutants. Chs3p is responsible for the synthesis of chitin dispersed in the cell wall, part of which is cross-linked to beta-1,3- and beta-1,6-glucan, as we showed previously. This chitin may be needed to strengthen the aberrant septa enough for completion of cytokinesis. The finding that ring contraction and primary septum formation are interdependent indicates that a previous genetic screen designed to identify proteins needed for Chs2p function is also a screen for proteins involved in contractile ring function. Indeed, the protein kinase Cla4p, earlier found in that screen, may act on the contractile ring, because Myo1p rings are abnormally wide in cla4 mutants. Two new proteins identified in the genetic screen, the protein kinase Cak1p and the septin Cdc11p, are being studied now. Cdc11p is a component of the microfilament ring found at the junction between mother and daughter cell and its involvement in septation is not unexpected, although its precise function was not previously elucidated.
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