Biosynthesis of lysosomal endopeptidases

AbstractDespite the clear differences between the amino acid sequence and enzymatic specificity of aspartic and cysteine endopeptidases, the biosynthetic processing of lysosomal members of these two families is very similar. With in vitro translation and pulse‐chase analysis in tissue culture cells, the biosynthesis of cathepsin D, a aspartic protease, and cathepsins B, H and L, cysteine proteases, are compared. Both aspartic and cysteine endopeptidases undergo cotranslational cleavage of an amino‐terminal signal peptide that mediates transport across the endoplasmic reticulum (ER) membrane. Addition of high‐mannose carbohydrate also occurs cotranslationally in the lumen of the ER. Proteases of both enzyme classes are initially synthesized as inactive proenzymes possessing amino‐terminal activation peptides. Removal of the propeptide generates an active single‐chain enzyme. Whether the single‐chain enzyme undergoes asymmetric cleavage into a light and a heavy chain appears to be cell type specific. Finally, late during their biosynthesis both classes of enzymes undergo amino acid trimming, losing a few amino acid residues at the cleavage site between the light and heavy chains and/