In spore forming microbes, germination protease (GPR) plays
a key role in the initiation of the germination process. A critical
step during germination is the degradation of small acidsoluble
proteins (SASPs), which protect spore DNA from external
stresses (UV, heat, low temperature, etc.). Inactive zymogen
GPR can be activated by autoprocessing of the N-terminal
pro-sequence domain. Activated GPR initiates the degradation
of SASPs; however, the detailed mechanisms underlying
the activation, catalysis, regulation, and substrate
recognition of GPR remain elusive. In this study, we determined
the crystal structure of GPR from Paenisporosarcina
sp. TG-20 (PaGPR) in its inactive form at a resolution of 2.5
Å. Structural analysis showed that the active site of PaGPR
is sterically occluded by an inhibitory loop region (residues
202–216). The N-terminal region interacts directly with the
self-inhibitory loop region, suggesting that the removal of the
N-terminal pro-sequence induces conformational changes,
which lead to the release of the self-inhibitory loop region
from the active site. In addition, comparative sequence and
structural analyses revealed that PaGPR contains two highly
conserved Asp residues (D123 and D182) in the active site,
similar to the putative aspartic acid protease GPR from Bacillus
megaterium. The catalytic domain structure of PaGPR
also shares similarities with the sequentially non-homologous
proteins HycI and HybD. HycI and HybD are metalloproteases
that also contain two Asp (or Glu) residues in their
active site, playing a role in metal binding. In summary, our
results
provide useful insights into the activation process of
PaGPR and its active conformation.