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1.  Crystallization of a nonclassical Kazal-type Carcinoscorpius rotundicauda serine protease inhibitor, CrSPI-1, complexed with subtilisin 
A recombinant serine protease inhibitor, CrSPI-1, from horseshoe crab, which inhibits subtilisin with a K i of 10−9  M, has been cloned, expressed, purified and cocrystallized with subtilisin. The crystals diffracted to 2.6 Å resolution.
Serine proteases play a major role in host–pathogen interactions. The innate immune system is known to respond to invading pathogens in a nonspecific manner. The serine protease cascade is an essential component of the innate immune system of the horseshoe crab. The serine protease inhibitor CrSPI isoform 1 (CrSPI-1), a unique nonclassical Kazal-type inhibitor of molecular weight 9.3 kDa, was identified from the hepatopancreas of the horseshoe crab Carcinoscorpius rotundicauda. It potently inhibits subtilisin and constitutes a powerful innate immune defence against invading microbes. Here, the cloning, expression, purification and cocrystallization of CrSPI-1 with subtilisin are reported. The crystals diffracted to 2.6 Å resolution and belonged to space group P21, with unit-cell parameters a = 73.8, b = 65.0, c = 111.9 Å, β = 95.4°. The Matthews coefficient (V M = 2.64 Å3 Da−1, corresponding to 53% solvent content) and analysis of the preliminary structure solution indicated the presence of one heterotrimer (1:2 ratio of CrSPI-1:subtilisin) and one free subtilisin molecule in the asymmetric unit.
PMCID: PMC2675604  PMID: 19407396
serine proteases; Kazal-type serine protease inhibitors; host–pathogen interaction; immune defence; horseshoe crab; CrSPI
2.  Structural basis for the methylation of G1405 in 16S rRNA by aminoglycoside resistance methyltransferase Sgm from an antibiotic producer: a diversity of active sites in m7G methyltransferases 
Nucleic Acids Research  2010;38(12):4120-4132.
Sgm (Sisomicin-gentamicin methyltransferase) from antibiotic-producing bacterium Micromonospora zionensis is an enzyme that confers resistance to aminoglycosides like gentamicin and sisomicin by specifically methylating G1405 in bacterial 16S rRNA. Sgm belongs to the aminoglycoside resistance methyltransferase (Arm) family of enzymes that have been recently found to spread by horizontal gene transfer among disease-causing bacteria. Structural characterization of Arm enzymes is the key to understand their mechanism of action and to develop inhibitors that would block their activity. Here we report the structure of Sgm in complex with cofactors S-adenosylmethionine (AdoMet) and S-adenosylhomocysteine (AdoHcy) at 2.0 and 2.1 Å resolution, respectively, and results of mutagenesis and rRNA footprinting, and protein-substrate docking. We propose the mechanism of methylation of G1405 by Sgm and compare it with other m7G methyltransferases, revealing a surprising diversity of active sites and binding modes for the same basic reaction of RNA modification. This analysis can serve as a stepping stone towards developing drugs that would specifically block the activity of Arm methyltransferases and thereby re-sensitize pathogenic bacteria to aminoglycoside antibiotics.
PMCID: PMC2896518  PMID: 20194115

Results 1-2 (2)