For more than 15 years, our group is extensively studying endosialidases, the specialized tailspike proteins of bacteriophages infecting the encapsulated bacterium Escherichia coli K1. Endosialidases specifically bind to and degrade the capsular polysaccharide of E. coli K1 – the polysialic acid (polySia) – a homopolymer composed of α2,8-linked sialic acid residues. These enzymes are also termed ‘endo-N-acyl-neuraminidases’ (endoN). Our expertise with endosialidases started in 1995 by cloning the first endoN gene (Gerardy-Schahn et al., 1995). Further studies focussed on maturation and proteolytic processing of the enzyme (Mühlenhoff et al., 2003; Schwarzer et al., 2007^I) as well as evolution of K1-phages, how they aquired their endoN gene (Stummeyer et al., 2006^I). In 2005, we could solve the first endoN crystal structure in collaboration with the laboratory of Prof. Ralf Ficner, Göttingen (Stummeyer et al., 2005). In collaboration with the laboratory of Prof. Michael Rossmann, Purdue - USA, the endoN structure could be inserted into the cryo-EM (cryo electron microscopy) map of the bacteriophage particle (Leiman et al., 2007). Additionally, together with the laboratory of Prof. Mark von Itzstein, Goldcoast - Australia , we have investigated binding and catalytic properties of endoN (Haselhorst et al., 2006; Schwarzer et al., 2009^I). Recently, we unravelled further atomic details of substrate binding sites (Schulz et al., 2010b; Schulz et al., 2010c) and the intramolecular C-terminal chaperone domain (Schulz et al., 2010a), which is required for proper folding and trimerisation of endoN and is proteolytically removed after completion of its job (Schwarzer et al., 2007^I). The new crystallographic data allowed us to elucidate the catalytic and the auto-proteolytic cleavage mechanism (Schulz et al., 2010a; Schulz et al., 2010c). By structure-based mutagenesis we created a set of endosialidase variants with different enzymatic activies and substrate binding capabilities (Schwarzer et al., 2009^I; Schulz et al., 2010c).

In total, endosialidases are important tools in polySia research as they can be used to specifically remove α2,8-linked polySia from any carrier (examples: Weinhold et al., 2005; Galuska et al., 2006; Oltmann-Norden et al., 2008^I; Rollenhagen*, Galuska* et al., 2010) or to hydrolyse artificial polySia hydrogels (Berski et al., 2008^I). As inactive variants, endosialidases are highly specific polySia lectins that bind polySia with high affinity of <2 nM (Schwarzer et al., 2009^I).

^I This work was financed and carried out in the frame of the interdisciplinary DFG Forschergruppe (FOR 548)

References

_{An asterix (*) indicates equal contribution of authors.}

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