Chitin synthase III requires Chs4p-dependent translocation of Chs3p into the plasma membrane

In Saccharomyces cerevisiae, Chs4p is required for chitin synthase III (CSIII) activity and hence for chitin synthesis. This protein is transported in vesicles in a polarized fashion independently of the other Chs proteins. Its association with membranes depends not only on prenylation, but also on its interaction with other proteins, mainly Chs3p, which is the catalytic subunit of CSIII and is able to properly direct Chs4p to the bud neck in the absence of prenylation. Chs4p is present in functionally limiting amounts and its overexpression increases Chs3p accumulation at the plasma membrane with a concomitant increase in chitin synthesis. In the absence of Chs4p, Chs3p is delivered to the plasma membrane but fails to accumulate there because it is rapidly endocytosed and accumulates in intracellular vesicles. A blockade of endocytosis stops Chs3p internalization, triggering a significant increase in chitin synthesis. This blockade is independent of Chs4p function, allowing the accumulation of Chs3p at the plasma membrane even in the chs4 mutant. However, the absence of Chs4p renders CSIII functionally inactive, independently of Chs3p accumulation at the plasma membrane. Chs4p thus promotes Chs3p translocation into the plasma membrane in a stable and active form. Proper CSIII turnover is maintained through the endocytic internalization of Chs3p.