Photolabelling of Salmonella typhimurium LT2 sialidase: Identification of a peptide with a predicted structural similarity to the active sites of influenza-virus sialidases

The sialidase from Salmonella tiphimurium LT2 was characterized by using photoaffinity-labelling techniques. The well-known sialidase inhibitor 5-acelamido-2,6-anhydro-3,5-dideoxy-D- glycero-D-galacta-non-2-enonic acid (NeuSAc2cn) was modified to contain an amino group at C-9, which permitted the incorporation of 4-azidosalicylic acid in amide linkage at this position. Labelling of the purified protein with the radioactive (¹²⁵I) photoprobe was determined to be highly specific for a region within the active-site cavity. This conclusion was based on the observation that the competitive inhibitor NeuSAc2en in the photolysis mixture prevented labelling of the protein. In contrast, compounds with structural and chemical features similar to the probe and NeuSAc2en, but which were not competitive enzyme inhibitors, did not affect the photolabelling of the protein. The peptide interacting with the probe was identified by CNBr treatment of the labelled protein, followed by N-terminal sequence analysis. Inspection of the primary structure of the protein, predicted from the cloned structural gene for the sialidase [Hoyer, Hamilton, Steenbergen and Vimr (1992) Mol. Microbiol. 6, 873 884] revealed that the label was incorporated into a 9.6 kDa fragment situated within the terminal third of the molecule near the C-terminal end. Secondary-structural predictions using the Garnier Robson algorithm [Garnier, Osguthorpe and Robson (1978) J. Mol. Biol. 120, 97 120] of the labelled peptide revealed a structural similarity to the active site of influenza-A- and Sendai-HN-virus sialidases with a repetitive series of alternating β-sheets connected with loops.