A Comparative Structural Analysis of the Flagellin Monomers of Caulobacter Crescentus Indicates That These Proteins Are Encoded by Two Genes

The flagellum of Caulobacter crescentus is composed of two flagellin polypeptide monomers which are distinguished by molecular weight and are closely related by biochemical and immunological criteria (C. Lagenaur and N. Agabian, J. Bacteriol. 132:731-733, 1977). The synthesis and assembly of these two flagellin proteins are developmentally regulated, and the periodicity of expression for each is distinct (C. Lagenaur and N. Agabian, J. Bacteriol. 135:1062-1069, 1978; M. A. Osley, M. Sheffery, and A. Newton, Cell 12:393-400, 1977). To understand the genetic and functional relationship between the 25,000- and 27,500-molecular-weight flagellins of C. crescentus CB15, a detailed comparative analysis of their protein structure was made, using a number of techniques, including one- and two-dimensional peptide mapping, a novel procedure of peptide alignment, and amino terminal amino acid sequence analysis. The tryptic peptides generated by each of the flagellins were compared by two-dimensional thin-layer chromatography. This peptide map analysis indicated that approximately 36% of the peptides generated from these two proteins had similar migration properties. Together with biochemical and immunological criteria, the two-dimensional peptide map suggested some structural relatedness between the monomers. However, a comparison of peptide fragments generated during partial protease digestion of each protein by a method of one-dimensional mapping indicated that the two proteins are structurally unique. A peptide alignment technique was developed to directly compare the primary structure of these proteins. In the peptide alignment procedure the amino terminus of each protein is radioactively labeled. After partial enzymatic digestion, the peptides are fractionated by polyacrylamide gel electrophoresis: those labeled at the amino terminus are then resolved by subsequent autoradiography. Each digest contains a family of amino-terminal-labeled fragments, the sizes of which reflect the sequential alignment of cleavage sites in the protein. A comparison of the alignment of specific cleavage sites of the two flagellins by this technique further established that each flagellin is structurally unique, particularly in the carboxyl terminal region. Finally, comparison of the amino terminal amino acid sequences indicated that the amino terminal region of both flagellins is highly conserved, but that the two polypeptides are clearly not identical. These findings strongly indicate that the two flagellins are encoded by distinct genetic loci and are not the product of novel processing of a single larger precursor.