Tolerance of the Ralstonia eutropha class I polyhydroxyalkanoate synthase for translational fusions to its C terminus reveals a new mode of functional display.

Here, the class I polyhydroxyalkanoate synthase (PhaC) from Ralstonia eutropha was investigated regarding the functionality of its conserved C-terminal region and its ability to tolerate translational fusions to its C terminus. MalE, the maltose binding protein, and green fluorescent protein (GFP) were considered reporter proteins to be translationally fused to the C terminus. Interestingly, PhaC remained active only when a linker was inserted between PhaC and MalE, whereas MalE was not functional. However, the extension of the PhaC N terminus by 458 amino acid residues was required to achieve a functionality of MalE. These data suggested a positive interaction of the extended N terminus with the C terminus. To assess whether a linker and/or N-terminal extension is generally required for a functional C-terminal fusion, GFP was fused to the C terminus of PhaC. Both fusion partners were active without the requirement of a linker and/or N-terminal extension. A further reporter protein, the immunoglobulin G binding ZZ domain of protein A, was translationally fused to the N terminus of the fusion protein PhaC-GFP and resulted in a tripartite fusion protein mediating the production of polyester granules displaying two functional protein domains.