Abstract
Endopeptidase-24.11 (EC 3.4.24.11), a widely distributed ectoenzyme, was isolated from pig kidneys by detergent solubilization of membranes and immuno-affinity chromatography. In all, 12 preparations of the enzyme were submitted to solid-phase sequencing, yielding a consensus sequence of 25 amino acid residues of the N-terminal segment. Some samples were treated with either trypsin or Staphylococcus aureus V8 proteinase before sequencing. There were four lysine and one arginine residues in the first nine positions. This segment was susceptible to hydrolysis by trypsin and, in some samples, to endogenous proteinases. From residue 19 onwards, the sequence became intensely hydrophobic. There was a striking homology with the N-terminal sequence of pro-sucrase-isomaltase. From Lys7 to Leu20 there were seven identical amino acid residues and four conservative substitutions. We suggest that endopeptidase-24.11 is topologically similar to this glycosidase, the N-terminus at the cytoplasmic face and hydrophobic segment serving the roles of both signal peptide and hydrophobic anchor.