Nature and Control of Cell Wall Biosynthesis

Abstract

The structural integrity of the skeletal cell wall is critical for the survival of fungi in their natural environments. The wall and outer surface are important in cell adhesion, cell shape maintenance and as a barrier to metabolites and drugs. The outer fuzzy layer of the wall is composed of proteins and mannoproteins and is involved in pathogenicity, phagocytosis and adherence. The exoskeleton is composed of polymers producing a scaffold that imparts rigidity and determines the cell shape. Adhesins have been identified on the outer cell surface and they interact with the host proteins laminin, fibrinogen and C3d of human complement. The s-glucans impart structural strength to the wall and at least three distinct glucans exist; a highly branched s1,6 glucan, a highly branced s1,3 glucan and an insoluble mixed s1,3/s1,6 glucan complexed to chitin. The chitin is linked to the mixed s-1,3/s-1,6 glucan through a 1,6 linkage of a glucose from the s1,3 glucan to a GlcNAc residue of the chitin. Wall fractions partially degraded by zymolyase were subjected to two-dimensional [1H] and [13C] NMR and the heteronuclear spectrum confirmed i) a pure glucan of glucose and ii) a s-linked polymer. A further zymolyase treatment produced a mixed s1,3/s1,6 glucan with mannoprotein bound to it. High resolution solid state [13C] NMR has been performed on the insoluble glucan and these data showed a s1,3 glucan with s1,6 side chains and a degree of polymerisation >400. Chitin and glucan are synthesized by transmembrane enzymes catalyzing the vectorial synthesis of these polymers and the products are extruded through into the wall. When the products emerge from the cell membrane they undergo intussusception into the expanding wall. At this point the molecules are linear and questions that remain unanswered include how the glucans are branched, cross linked with other polymers such as chitin and complexed with mannoproteins. Glucanases have been implicated in the controlled hydrolytic modification of wall glucans for morphogenesis and development of the final wall structure. A secreted s-glucan branching enzyme from C. albicans has recently been described. A model for cell wall growth and development will be presented.