Autocatalytic activation of human legumain at aspartic acid residues.

Human legumain was characterized following overexpression in a murine cell line as the C-terminal Ig-fusion protein. Upon acid treatment, the prolegumain autoproteolyzed distal to two aspartic acid residues to yield a highly active form. The ability of mature legumain to cleave after aspartic acid residues was confirmed with a small peptide substrate. Substitution of alanine for the putative catalytic cysteine, or for either of two strictly conserved histidine residues, partly or wholly eliminated autoactivation but not the ability of wild-type legumain to correctly process the variants to the properly sized proteins.