Zymocin, alpha subunit. Zymocin is a heterotrimeric enzyme that inhibits yeast cell cycle progression. The zymocin alpha subunit has a chitinase activity that is essential for holoenzyme action from the cell exterior while the gamma subunit contains the intracellular toxin responsible for G1 phase cell cycle arrest. The zymocin alpha and beta subunits are thought to act from the cell's exterior by docking to the cell wall-associated chitin, thus mediating gamma-toxin translocation. The alpha subunit has an eight-stranded TIM barrel fold similar to that of family 18 glycosyl hydrolases such as hevamine, chitolectin, and chitobiase.