Recombinant expression and characterization of an epididymis-specific antimicrobial peptide BIN1b/SPAG11E

BIN1b was reported as an epididymis-specific beta-defensin antimicrobial peptide. In this paper, the recombinant BIN1b was expressed and purified by fusing with GB1-His tag. The size-exclusion gel filtration experiment indicated that the fusion protein GB1-BIN1b formed multimers at pH 7.4, and existed as monomer at pH 4.5. The oligomerization of GB1-BIN1b was only related to pH value, neither to NaCl concentration nor protein concentration. Far-UV circular dichroism (CD) spectra also showed the fusion protein had more ordered secondary structures at pH 4.5 than at pH 7.4, as a negative peak appeared around 218 nm indicative of typical β-sheet. The 2D 15N-1H heteronuclear single-quantum coherence (HSQC) spectra suggested that the fusion protein adopted a compact three-dimensional structure at pH 4.5. Colony forming unit (CFU) inhibition assay demonstrated that 25 μM fusion protein at pH 7.4 had an antimicrobial activity of 40% against E. coli K12D31, which might imply the fusion protein functions as multimeric states. In conclusion, the GB1 fusion partner helps BIN1b form a stable homogenous conformation to facilitate subsequent structural determination without a significant effect on the antimicrobial activity.