Conformational folding and stability of the HET-C2 glycolipid transfer protein fold: does a molten globule-like state regulate activity?

The glycolipid transfer protein (GLTP) superfamily is defined by the human GLTP fold that represents a novel motif for lipid binding and transfer and for reversible interaction with membranes, i.e., peripheral amphitropic proteins. Despite limited sequence homology with human GLTP, we recently showed that HET-C2 GLTP of Podospora anserina is organized conformationally as a GLTP fold. Currently, insights into the folding stability and conformational states that regulate GLTP fold activity are almost nonexistent. To gain such insights into the disulfide-less GLTP fold, we investigated the effect of a change in pH on the fungal HET-C2 GLTP fold by taking advantage of its two tryptophans and four tyrosines (compared to three tryptophans and 10 tyrosines in human GLTP). pH-induced conformational alterations were determined by changes in (i) intrinsic tryptophan fluorescence (intensity, emission wavelength maximum, and anisotropy), (ii) circular dichroism over the near-UV and far-UV ranges, including thermal stability profiles of the derivatized molar ellipticity at 222 nm, (iii) fluorescence properties of 1-anilinonaphthalene-8-sulfonic acid, and (iv) glycolipid intermembrane transfer activity monitored by Förster resonance energy transfer. Analyses of our recently determined crystallographic structure of HET-C2 (1.9 Å) allowed identification of side chain electrostatic interactions that contribute to HET-C2 GLTP fold stability and can be altered by a change in pH. Side chain interactions include numerous salt bridges and interchain cation-π interactions, but not intramolecular disulfide bridges. Histidine residues are especially important for stabilizing the local positioning of the two tryptophan residues and the conformation of adjacent chains. Induction of a low-pH-induced, molten globule-like state inhibited glycolipid intermembrane transfer by the HET-C2 GLTP fold.