Aminoacylase I from porcine kidney: identification and characterization of two major protein domains.

The domain structure of hog-kidney aminoacylase I was studied by limited proteolytic digestion with trypsin and characterization of the resulting fragments. In the native enzyme, the sequences from residue 6 to 196 and 307 to 406 are resistant to trypsin and remain tightly bound in nondenaturing solvents, while the intervening sequence (197-306) is efficiently degraded by trypsin. We conclude that the N-terminal half of the molecule and its C-terminal fourth form two independently folded domains. Both contain a peculiar PWW(A,L) sequence motif preceded by several strongly polar residues. We propose that these sequences form surface loops that mediate the membrane association of aminoacyclase I. We further show that the three free cysteine residues and the essential Zn2+ ion reside in the trypsin-resistant domains, while the intervening sequence contains the only disulfide H bond of the protein.