B. Lenarcic et al., Saxiphilin, a saxitoxin-binding protein with two thyroglobulin type 1 domains, is an inhibitor of papain-like cysteine proteinases, J BIOL CHEM, 275(20), 2000, pp. 15572-15577
The type 1 domain of thyroglobulin is a protein module (Thyr-1) that occurs
in a variety of secreted and membrane proteins. Several examples of Thyr-1
modules have been previously identified as inhibitors of the papain family
of cysteine proteinases, Saxiphilin is a neurotoxin-binding protein from b
ullfrog and a homolog of transferrin with a pair of such Thyr-1 modules loc
ated in the N-lobe. Saxiphilin is now characterized as a potent inhibitor o
f three cysteine proteinases as follows: papain, human cathepsin B, and cat
hepsin L, The stoichiometry of enzyme inhibition reveals that both Thyr-1 d
omains of saxiphilin inhibit papain (apparent K-i = 1.72 naa), but only one
of these domains inhibits cathepsin B (K-i = 1.67 nM) and cathepsin L (K-i
= 0.02 nM). Physical association of saxiphilin and papain blocked from tur
nover at the active-site cysteine residue can be detected by cross-linking
with glutaraldehyde. The rate of association of saxiphilin and cathepsin B
is strongly pH-dependent with an optimum at pH 5.2, reflecting control by a
t least two H+-titratable groups. These results further demonstrate that va
rious Thyr-1 domains are selective inhibitors of cysteine proteinases with
utility in the study of protein interactions and degradation.