Abstract
Neurospora crassa produces a phase-specific cationic mucopolysaccharide composed primarily of galactosamine (galactosaminoglycan mucopolysaccharide; GAG-MP), which becomes part of the cell wall and is later secreted into the medium. Its appearance coincides with the onset of the restricted phase of growth, and causes efflux of small molecular weight metabolites when incubated with conidial cells. This activity may be a product of electrostatic interactions of the GAG-MP with the conidial plasma membrane. The activity is blocked by acetylation of the primary amines on the molecule, digestion by enzymes that hydrolyse carbohydrate linkages and the inclusion of NaCl (1 m) in the GAG-MP/conidia reaction mixture. The physiological activity of the GAG-MP mimics that of known depolarizing agents. A model is proposed in which GAG-MP depolarizes the plasma membrane at the onset of the restricted phase of growth, stimulating a cell surface enzyme to produce endogenous cyclic AMP which in turn switches on the enzymic and genetic machinery necessary for that phase of growth.
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