An atomic force microscopy analysis of yeast mutants defective in cell wall architecture

Yeast cells are surrounded by a thick cell wall, the composition and structure of which have been characterized by biochemical and genetic methods. In this study, we used atomic force microscopy (AFM) to visualize the cell surface topography and to determine cell wall nanomechanical properties of yeast mutants defective in cell wall architecture. While all mutants investigated showed some alteration in cell surface topography, this alteration was particularly salient in mutants defective in beta-glucan elongation (gas1), chitin synthesis (chs3) and cross-linkages between chitin and beta-glucan (crh1crh2). In addition, these alterations in surface topology were accompanied by increased roughness of the cell. From force-indentation curves, the Young's modulus was determined, as it gives a measure of the elasticity of the cell wall. A value of approximately 1.6 MPa was obtained for the cell walls of the wild-type strain in exponential and stationary phases of growth. The same value was measured in a mnn9 mutant defective in protein mannosylation, and was two-fold reduced in a mutant with reduced beta-glucan (fks1Delta and knr4Delta), only in the stationary phase of growth. In contrast, the elasticity was dramatically reduced in mutants defective in chitin synthesis (chs3Delta), beta-glucan elongation (gas1Delta) and, even more remarkably, in a crh1Deltacrh2Delta mutant defective in the enzymes that catalyse cross-linkages of chitin to beta-glucan. Taken together, these results provide direct physical evidence that the nanomechanical properties of the yeast cell wall are mainly dependent on cross-links and cell wall remodelling, rather than on cell wall composition or thickness.