Role of the lipase-specific foldase of Burkholderia glumae as a steric chaperone.

Most lipases of Gram-negative bacteria require a lipase-specific foldase (Lif) in order to fold in the periplasm into their active, protease-resistant conformation prior to their secretion. The periplasmic domain of the Lif (amino acids 44-353) of Burkholderia glumae was purified as a His-tagged protein, and its function in the folding of lipase was studied in vitro. Refolding of the denatured lipase into its active conformation was dependent on the presence of the Lif. Circular dichroism revealed that the lipase refolded in the absence of Lif into a form with a native-like conformation, which was more stable against heat-induced denaturation than the native form, but was enzymatically inactive. This form of the protein could be activated by adding Lif after several hours, which demonstrates that the function of this chaperone is to help lipase to overcome an energetic barrier in the productive folding pathway rather than to prevent it from entering a non-productive pathway. The Lif was shown to interact with the native lipase in protease-protection experiments as well as by affinity chromatography, consistent with a role of the Lif late in the folding process. These results demonstrate that the Lif functions in a way analogous to the propeptides of many bacterial proteases and indicate that the amino acid sequence of the lipase does not contain all the information required for the protein to adopt its three-dimensional structure.