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

From the ¹ From the Department of Physiological Chemistry, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205 and Department of Biochemistry, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261

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.

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