Influence of N-Terminal Truncations on the Functional Expression of Bacillus licheniformis γ-Glutamyltranspeptidase in Recombinant Escherichia coli

The full-length Bacillus licheniformis gamma-glutamyltranspeptidase (BlGGT) gene and six truncations lacking 36, 129, 132, 135, 144, and 174 bp, respectively, at the 5' end were prepared by polymerase chain reaction and cloned into the expression vector pQE-30. Isopropyl-beta-d-thiogalactopyranoside induction of Escherichia coli M15 cells bearing the recombinant plasmids resulted in the overexpression of His(6)-tagged proteins BlGGT, BlGGT/Delta N12, BlGGT/Delta N43, BlGGT/Delta N44, BlGGT/Delta N45, BlGGT/Delta N48, and BlGGT/Delta N58. Except for BlGGT/Delta N58, the overexpressed enzymes could be purified to near-homogeneity by Ni2+-NTA resin. The molecular masses of the precursor and subunits of BlGGT, BlGGT/Delta N12, and BlGGT/Delta N43 were determined to be 63, 41, and 22 kDa, respectively, by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, but other recombinant enzymes exhibited predominantly as a precursor form. The specific activity for purified BlGGT, BlGGT/Delta N12, BlGGT/Delta N43, and BlGGT/Delta N44 was 51.9 +/- 5.6, 1.3 +/- 0.2, 0.8 +/- 0.05, and 0.2 +/- 0.03 U/mg protein, respectively, whereas the remaining two enzymes had shown no GGT activity under the enzyme assay conditions. BlGGT, BlGGT/Delta N12, BlGGT/Delta N43, and BlGGT/Delta N44 could process autocatalytically their precursors into alpha- and beta-subunits at 4C. These results indicate that removal of the signal peptide significantly affects the functional expression of BlGGT in recombinant E. coli.