Aspartic proteinase A from yeast is specifically and potently inhibited by
a small protein called IA(3) from Saccharomyces cerevisiae. Although this i
nhibitor consists of 68 residues, we show that the inhibitory activity resi
des within the N-terminal half of the molecule. Structures solved at 2.2 an
d 1.8 Angstrom, respectively, for complexes of proteinase A with full-lengt
h IA(3) and with a truncated form consisting only of residues 2-34, reveal
an unprecedented mode of inhibitor-enzyme interactions, Neither form of the
free inhibitor has detectable intrinsic secondary structure in solution. H
owever, upon contact with the enzyme, residues 2-32 become ordered and adop
t a near-perfect alpha-helical conformation. Thus, the proteinase acts as a
folding template, stabilizing the helical conformation in the inhibitor, w
hich results in the potent and specific blockage of the proteolytic activit
y.