4.D.1.1.12
Chitin synthase 3, Chs3, of 1165 aas and 7 TMSs in a 2 + 1 + 4 TMS arrangement. Chitin biosynthesis in yeast is accomplished by three chitin synthases (Chs) termed Chs1 (TC# 4.D.1.1.17), Chs2 (TC# 4.D.1.1.16) and Chs3, the last which accounts for most of the chitin deposited within the cell wall. While the overall structures of Chs1 and Chs2 are similar to those of other chitin synthases from fungi and arthropods, Chs3 lacks some of the C-terminal transmembrane helices raising questions regarding its structure and topology. Gohlke et al. 2017 determined aspects of the catalytic domain, the chitin-translocating channel and the interfacial helices in between. They identified an amphipathic, crescent-shaped alpha-helix attached to the inner side of the membrane that may control the channel entrance and a finger helix pushing the polymer into the channel. Chitin synthases form oligomeric complexes, which may be necessary for the formation of chitin nanofibrils. They detected oligomeric complexes at the bud neck, the lateral plasma membrane, and in membranes of Golgi vesicles (Gohlke et al. 2017). The combined action of two independent but redundant endocytic recycling mechanisms, together with distinct labels for vacuolar degradation, determines the final fate of the intracellular traffic of the Chs3 protein, allowing yeast cells to regulate morphogenesis, depending on environmental constraints (Arcones et al. 2016).