Regulation of chitin synthesis during growth of fungal hyphae: the possible participation of membrane stress

Abstract

Apical hyphal extension involves very localized apical deposition of newly synthesized wall skeletal material, notably chitin. A branch forms where a new localized site of chitin deposition occurs in the lateral wall. Key enzymes involved are the chitin synthases. Their activity must be under tight regulation to achieve the orderly deposition of chitin. There is evidence that inactive chitin synthase is distributed throughout the hyphal plasma membrane and activated at the apex and at an incipient branch site. At these sites, the wall is plastic. We have investigated the hypothesis that physical stressing of the membrane, a consequence of the cell's turgor pressure acting at these weaker points, may locally activate the chitin synthase. Results show that cells that have been subjected to hypoosmotic stress have raised native chitin synthase activities. It is suggested that stressing the membrane may cause a conformational change in chitin synthase molecules in the membrane or changes in the interactions between chitin synthase and associated polypeptides, leading to activation. This process may act along with other regulatory mechanisms discussed here, such as post-translational modification and availability of allosteric effectors, to restrict the enzymic activity to sites where chitin synthesis is required. Key words: chitin synthase, zymogen, turgor pressure, membrane stress, Candida albicans, hyphal growth.