Protein PAB, a mosaic albumin-binding bacterial protein representing the first contemporary example of module shuffling.

Some strains of the anaerobic human commensal and pathogen Peptostreptococcus magnus bind human serum albumin (HSA), whereas other strains of this species express protein L, an immunoglobulin light chain-binding surface protein. A novel HSA-binding protein called protein PAB was purified in one step from the culture supernatant of an HSA-binding strain of P. magnus by affinity chromatography on HSA-Sepharose. The apparent size of the molecular was 47 kDa on SDS-polyacrylamide gel electrophoresis. Amino acid sequence analysis of protein PAB demonstrated that the 4 NH2-terminal residues were identical to the corresponding sequence in protein L. In a polymerase chain reaction, oligonucleotides based on extragenic 5'- and 3'-end sequences of the protein L gene generated a product of the expected size: 1.3 kilobase pairs. A recombinant protein with retained albumin binding capacity was expressed in Escherichia coli, and the nucleotide sequence of the protein PAB gene was determined. The structural gene is 1161 nucleotides long, corresponding to a preprotein of 387 amino acids and a molecular mass of 43,043 Da. Unlike most other Gram-positive bacterial surface proteins described, protein PAB contains no internal homologies. However, substantial homologies were found to both proteins L and G (the IgG- and HSA-binding surface protein of group C and G streptococci). The derived amino acid sequence of the 135-base pair-long region homologous to protein G corresponds to the HSA-binding domain of that protein, and in protein PAB, this region is inserted between sequences showing extensive homology to COOH-terminal regions of peptostreptococcal protein L. This mosaic organization of protein PAB demonstrates that the molecule is a product of intergenic interspecies recombination of a functional domain into a common framework for peptostreptococcal surface proteins. Such an interspecies exchange of a functional protein module has previously not been described in prokaryotic cells.