Chitin Synthetases 1 and 2 from Yeast, Two Isoenzymes with Different Functions

Abstract

Chitin, a linear polymer of β(1–4)-linked N-acetylglucosamine, is a major structural component of the cell wall and septum in many fungi. In Saccharomyces cerevisiae, its localization is restricted to a specific area, a circumstance that facilitates studies of its function. Although some of the yeast chitin is found in the cell wall, most of it is in the bud scars that remain on the surface of the mother cell after the daughter cell has separated. That distribution can be traced back to the deposition of chitin during the cell cycle (Cabib et al., 1982): a ring of chitin appears at the neck between mother and daughter cell in the early stages of budding (Fig. 1); later, at cytokinesis, as the plasma membrane invaginates, more chitin is laid down to form a disc, the primary septum, that closes the channel between the two cells. This is followed by the laying down of secondary septa between which the chitinous primary septum becomes sandwiched (Fig. 1). Finally, the two cells separate along the line of the primary septum, with most of the chitin remaining in the bud scar on the mother cell (Fig. 1).