Functional analysis of the large periplasmic loop of the Escherichia coli K-12 WaaL O-antigen ligase

José M. Pérez, Megan A. McGarry, Cristina L. Marolda, Miguel A. Valvano

Research output: Contribution to journalArticlepeer-review

60 Citations (Scopus)


WaaL is a membrane enzyme implicated in ligating undecaprenyl-diphosphate (Und-PP)-linked O antigen to lipid A-core oligosaccharide. We determined the periplasmic location of a large (EL5) and small (EL4) adjacent loops in the Escherichia coli K-12 WaaL. Structural models of the EL5 from the K-12, R1 and R4 E. coli ligases were generated by molecular dynamics. Despite the poor amino acid sequence conservation among these proteins, the models afforded similar folds consisting of two pairs of almost perpendicular α-helices. One α-helix in each pair contributes a histidine and an arginine facing each other, which are highly conserved in WaaL homologues. Mutations in either residue rendered WaaL non-functional, since mutant proteins were unable to restore O antigen surface expression. Replacements of residues located away from the putative catalytic centre and non-conserved residues within the centre itself did not affect ligation. Furthermore, replacing a highly conserved arginine in EL4 with various amino acids inactivates WaaL function, but functionality reappears when the positive charge is restored by a replacement with lysine. These results lead us to propose that the conserved amino acids in the two adjacent periplasmic loops could interact with Und-PP, which is the common component in all WaaL substrates.

Original languageEnglish
Pages (from-to)1424-1440
Number of pages17
JournalMolecular Microbiology
Issue number6
Publication statusPublished - Dec 2008

ASJC Scopus subject areas

  • Microbiology
  • Molecular Biology


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