The Role of Polysaccharide in the Secretion of Protein by Micrococcus sodonensis

Cell suspensions of Micrococcus sodonensis (ATCC 11880) secrete 7 to 10 individual proteins including an alkaline phosphatase, a nuclease, and a protease. The appearance of enzyme activities in the extracellular medium is dependent on the co-secretion of at least one of several polysaccharides also elaborated by these cells. This conclusion is based on the following observations: (a) bacitracin, an inhibitor of polysaccharide biosynthesis, proportionately inhibits polysaccharide and protein secretion at concentrations which have no effect on intracellular protein synthesis; (b) a mutant, impaired in its ability to secrete proteins, is also impaired in its ability to secrete a glucosamine-rich polysaccharide; (c) alkaline phosphatase secretion by mutant protoplasts is enhanced in the presence of exogenously supplied polysaccharide; (d) inclusion of 10 mm glucosamine or 2-deoxyglucose (a variety of other monosaccharides were ineffective) in the medium suppresses the secretion of all three enzymes as well as the glucosamine-containing polysaccharide; and (e) restoration of alkaline phosphatase secretion in glucosamine-inhibited protoplasts is achieved by the addition of purified preparations of the glucosamine-containing polysaccharide. The inhibition of enzyme secretion by either glucosamine or 2-deoxyglucose can be completely prevented or reversed by addition of equimolar concentrations of glucose. The addition of albumin or casein to glucosamine-inhibited cells markedly enhances the rate of alkaline phosphatase secretion. These results suggest that during secretion polypeptide chains are vulnerable to proteolytic degradation and that co-secretion of polysaccharide protects the protein from proteolysis.