Gf. Mcintyre et al., THE PH-DEPENDENT MEMBRANE ASSOCIATION OF PROCATHEPSIN-L IS MEDIATED BY A 9-RESIDUE SEQUENCE WITHIN THE PROPEPTIDE, The Journal of biological chemistry, 269(1), 1994, pp. 567-572
The lysosomal proprotease procathepsin L binds to mouse fibroblast mic
rosomal membranes at pH 5, but mature active cathepsin L does not (McI
ntyre, G. F., and Erickson, A. H. (1991) J. Biol. Chem. 266, 15438-154
45). This binding is not dependent on N-linked carbohydrate as procath
epsin L synthesized in cells treated with tunicamycin still shows pH-d
ependent membrane association. These results suggest that the propepti
de (Thr18-Lys113) of the cysteine protease mediates its pH-dependent m
embrane association. Synthetic peptides containing either 24 or 9 resi
dues from the N-terminal portion of the mouse procathepsin L propeptid
e inhibited the binding of mouse procathepsin L to microsomal membrane
s at pH 5. In contrast, the pH-dependent membrane association was not
inhibited either by a scrambled version of the 24-residue peptide, in
which 3 adjacent residues likely to be positively charged at pH 5 were
dispersed, or by a second control peptide containing the 11 N-termina
l residues from mature mouse cathepsin L. The 24-residue peptide chemi
cally coupled to horseradish peroxidase bound to microsomes at pH 5, b
ut not at pH 7. On ligand blots, the same conjugate bound specifically
to a 43-kDa integral membrane protein, identifying the microsomal pro
tein that mediates the proenzyme binding. The 9-residue propeptide seq
uence that inhibits the membrane association of procathepsin L at pH 5
resembles the vacuolar sorting sequences in the propeptides of yeast
proteinase A and carboxypeptidase Y. This suggests that the membrane a
ssociation of procathepsin L may play a role in the transport of the p
roenzyme to lysosomes, the vacuolar equivalent in mammalian cells.