Saccharomyces cerevisiae cell wall chitin, the Kluyveromyces lactis zymocin receptor

The exozymocin secreted by Kluyveromyces lactis causes sensitive yeast cell s, including Saccharomyces cerevisiae, to arrest growth in the G, phase of the cell cycle. Despite its heterotrimeric (alpha beta gamma) structure, in tracellular expression of its smallest subunit, the gamma -toxin, is alone responsible for the G(1) arrest. The alpha subunit, however, has a chitinas e activity that is essential for holozymocin action from the cell exterior. Here we show that sensitive yeast cells can be rescued from zymocin treatm ent by exogenously applying crude chitin preparations, supporting the idea that chitin polymers can compete for binding to zymocin with chitin present on the surface of sensitive yeast cells. Consistent with this, holozymocin can be purified by way of affinity chromatography using an immobilized chi tin matrix. PCR-mediated deletions of chitin synthesis (CHS) genes show tha t most, if not all, genetic scenarios that lead to complete loss (chs Delta ), blocked export (ells7 Delta) or reduced activation (chs4 Delta), combine d with mislocalization (chs4 Delta chs5 Delta; chs4 Delta chs6 Delta; chs4 Delta chs5 Delta chs6 Delta) of chitin synthase III activity (CSIII), rende r cells refractory to the inhibitory effects of exozymocin. In contrast, de letions in CHS1 and CHS2, which code for CSI and CSII, respectively, have n o effect on zymocin sensitivity. Thus, CSIII-polymerized chitin, which amou nts to almost 90%,, of the cell's chitin resources, appears to be the carbo hydrate receptor required for the initial interaction of zymocin with sensi tive cells. Copyright (C) 2001 John Wiley & Sons, Ltd.