The lectin-like properties of the extracellular protein produced by Pseudomonas aeruginosa during hexadecane degradation

Abstract

Numerous microorganisms can degrade hydrocarbons and many produce extracellular compounds. These compounds are generally thought to emulsify hydrocarbons making them more available to the microorganism and stimulating growth. Pseudomonas aeruginosa produces both a rhamnolipid surfactant and an extracellular protein during growth on n-hexadecane (C$\rm\sb{16}H\sb{34}).$ The protein has been hypothesized to stimulate growth by emulsifying hexadecane. However, it has never been shown that the protein has hydrophobic properties characteristic of emulsifiers. An isolation procedure was developed in this study that produces 20-30 mg of very pure protein per 500 ml culture of strain ATCC 17423. The protein is monomeric with a molecular weight of approximately 14,500 determined by SDS-PAGE. Structurally the protein is similar to two other proteins from strains S7B1 and PG210. The proteins from strain S7B1 and 17423 stimulate growth of P. aeruginosa on hexadecane. The hydropathic index of the protein from strain PG201 shows no strong hydrophobic regions in the amino acid sequence. Also the isolated protein from strain 17423 will not bind during hydrophobic chromatography and always acts as a monomer in solution even at concentrations which should cause hydrophobic proteins to aggregate. These results indicate that the protein is not hydrophobic and therefore does not have surfactant-like properties. A surprising result from this investigation is that the protein from strain 17423 has lectin-like (carbohydrate binding) qualities and agglutinates P. aeruginosa, Escherichia coli, human type O, and horse red blood cells. The agglutination is inhibited by EDTA, glucose, mannose and rhamnose. The exact carbohydrate(s) the protein binds has not been determined but evidence suggests that it may bind the carbohydrate portion of the rhamnolipid surfactant. A new model is presented describing the function of the protein. In this model the extracellular protein functions by binding the emulsified hydrocarbon to the outer membrane of the bacterium by both the carbohydrate on the glycolipid surfactant and the lipopolysaccharide of the bacterium. This binding of the hydrocarbon stimulates growth of the bacterium on hexadecane.