Chitin synthases in yeast and fungi.

The polysaccharide chitin is an important structural component of the cell walls of many fungi. Chitin synthesis is directly governed by an enzymatic activity called chitin synthase (CS). The use of the budding yeast Saccharomyces cerevisiae as a biological model allowed the identification of three distinct chitin synthase activities: CSI, involved in repair functions at the end of cytokinesis; CSII, responsible for the synthesis of the primary septum that separates mother and daughter cells; and CSIII, responsible for the formation of the ring (bud scar) where most of the cell wall chitin is located. These chitin synthases differ not only in functions but also in catalytic properties. The catalytic subunit of each of these activities is encoded by separated genes, CHS1, CHS2 and CHS3, respectively, although it has been shown in S. cerevisiae that CSIII activity also depends on the products of other genes. To date, several chitin synthase (CHS) genes have been also identified in other fungi; most of them are similar to ScCHS1 and ScCHS2 genes and are classified in chitin synthases classes I, II and III in terms of sequence similarity. The rest are defined as two CHS classes, IV and V, highly similar to ScCHS3. While CHS class V genes have been only identified in filamentous fungi and their functions are unknown, class IV genes, which includes ScCHS3, are involved in the synthesis of most chitin in yeast cells.