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Background  The three bacteriophage genes  gtrA, gtrB  and  gtr (type)   are responsible for O-antigen glucosylation in  Shigella flexneri . Both  gtrA  and  gtrB  have been demonstrated to be highly conserved and interchangeable among serotypes while  gtr (type)   was found to be specific to each serotype, leading to the hypothesis that the Gtr (type)  proteins are responsible for attaching glucosyl groups to the O-antigen in a site- and serotype- specific manner. Based on the confirmed topologies of GtrI, GtrII and GtrV, such interaction and attachment of the glucosyl groups to the O-antigen has been postulated to occur in the periplasm.    Results  In this study, the topology of GtrIV was experimentally determined by creating different fusions between GtrIV and a dual-reporter protein, PhoA/LacZ. This study shows that GtrIV consists of 8 transmembrane helices, 2 large periplasmic loops, 2 small cytoplasmic N- and C- terminal ends and a re-entrant loop that occurs between transmembrane helices III and IV. Though this topology differs from that of GtrI, GtrII, GtrV and GtrX, it is very similar to that of GtrIc. Furthermore, both the N-terminal periplasmic and the C-terminal periplasmic loops are important for GtrIV function as shown via a series of loop deletion experiments and the creation of chimeric proteins between GtrIV and its closest structural homologue, GtrIc.    Conclusion  The current study provides the basis for elucidating the structure and mechanism of action of this important O-antigen modifying glucosyltransferase.

Topological characterisation and identification of critical domains within glucosyltransferase IV (GtrIV) of Shigella flexneri