Caulobacter crescentus pilin. Purification, chemical characterization, and NH2-terminal amino acid sequence of a structural protein regulated during development.

The pili of Caulobacter crescentus are assembled during swarmer cell development at the differentiating cell pole. Under specific growth conditions, it was found that C. crescentus CB15 will produce insoluble pigmented granules which entrap pili lost during cell growth along with other extracellular proteins. This provided a strategy for protein purification. Pilin was purified from these granules by gel filtration in the presence of high levels of detergent. A protein of the appropriate molecular weight for pilin was isolated by this procedure and demonstrated to be pilin by specific labeling of the intact pilus with ferritin-coupled antibodies. CB15 pilin has an apparent molecular weight of 8,000, is rich in hydrophobic amino acids (greater than 70%), and has a broad isoelectric focusing range centered about a pI of 6.6. Periodic acid-Schiff reagent staining did not demonstrate carbohydrate modification of the monomer. The pilins of both CB15 and a related strain CB13 cross-react with anti-CB15 pilin antibody. Both strains also demonstrate similar RNA bacteriophage sensitivity, although there were significant differences in the amino acid composition, molecular weight, and other physical properties of the pilins from these two related strains. The NH2-terminal amino acid sequence of about 40% of the CB15 pilin molecular has been determined and bears some resemblance to that of some common pili; however, the sequences are not homologous and there is no indications of an unusual NH2-terminal amino acid in Caulobacter pilin.