K. Nomura et al., SEA-URCHIN HATCHING ENZYME (ENVELYSIN) - CDNA CLONING AND DEPRIVATIONOF PROTEIN SUBSTRATE-SPECIFICITY BY AUTOLYTIC DEGRADATION, Biochemistry, 36(23), 1997, pp. 7225-7238
The hatching enzyme (envelysin) of the sea urchin Hemicentrotus pulche
rrimus was purified from the medium of hatched blastulae. By cDNA clon
ing its deduced amino acid sequence and molecular architecture were re
vealed. The 591-residue precursor with calculated M-r of 66 123 consis
ts of an 18-residue signal sequence, a 151-residue propeptide, and a 4
22-residue mature enzyme with N-terminal catalytic and C-terminal hemo
pexin-like domains. As compared with that of Paracentrotus lividus, it
s amino acid sequence is 69% identical and 10% similar. They share typ
ical structural features with the mammalian MMP gene family members: c
ysteine switch, zinc-binding signature, methionine-turn, Cys residues
near both ends of hemopexin-like domain, etc. However, its propeptide
has a 70-residue extra sequence with an Asp- and Glu-rich stretch, sup
posedly involved in the proenzyme activation by binding Ca2+ ions in s
eawater. The hinge region is also longer than those of most MMPs, with
an extra sequence rich in Thr and Arg residues. Mature 50K enzyme is
highly susceptible to autolytic cleavage at Gln(503)-Leu(504), produci
ng the 38K form retaining catalytic activity and substrate specificity
against fertilization envelope. The 38K form and 15K fragment were co
eluted from a gel-filtration column, suggesting that these two fragmen
ts are disulfide-bridged and that the tertiary structure is not much d
eviated. The 38K form further autolyzed to 32K form by cleaving Tyr(45
0)-Tyr(451) bond with the loss of protein-substrate specificity, retai
ning only nonspecific protease activity. Thus, the autolytic release o
f 2/3 of the C-terminal domain reduced the highly specific enzyme to a
common nonspecific protease, implying that the size and structure of
almost the entire hemopexin-like domain is essential for the protein s
ubstrate specificity. Moreover, autolytic degradation of envelysins fr
om the two species follow quite different pathways despite their high
homology in structure. The 38K and 32K forms were inhibited by bovine
TIMP-1 with different IC50 values, indicating that its inhibitory acti
vity depends on the extent of the interaction with the C-terminal doma
in of the enzyme.