Here we present a continuation study of our previous paper, Spontaneous Formation of Cushioned Model Membranes Promoted by an Intrinsically Disordered Protein, in which we found that upon the interaction of Histatin 5 (Hst5) with a model lipid bilayer, a peptide cushion spontaneously forms underneath the bilayer, without disrupting the integrity of the bilayer. Hst5 is a histidine-rich, intrinsically disordered, multifunctional saliva peptide known to act as a first line of defense against oral candidiasis caused by Candida albicans. This study aimed to test our hypothesis that this effect is of electrostatic origin and that the observed behavior is due to proton charge fluctuations of the histidines, promoting attractive electrostatic interactions between the positively charged proteins and the anionic surfaces, with concomitant counterion release.

Here, we are investigating the role of the histidines in more detail by defining a library of variants of the peptide, where the pH-insensitive amino acid glutamine has replaced the former.

Figure 1: Schematic representation of where in the system the peptides end up after interaction with the lipid bilayer, depending on the number of histidines in the sequence.

The peptides were characterized in bulk using circular dichroism (CD) and small angle x-ray scattering (SAXS). Supported lipid bilayers (POPC:POPS 9:1) were deposited on SiO₂ surfaces using vesicle fusion, and the bilayer-peptide interactions were studied using neutron reflectometry (NR), as well as quartz-crystal microbalance with dissipation monitoring (QCM-D).

There are still more properties of Hst5 we need to investigate to understand how the peptide can translocate the bilayer without disrupting its integrity. We are currently in the process of investigating the properties of the sequence, such as the order of the amino acids, as well as the length of the peptide chain.

The work was done in collaboration between researchers at Lund University, Institut Laue–Langevin, and CNR Institute for Complex Systems. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 101004806, MOSBRI-2021-29, Sweden’s Innovation Agency (Vinnova), and the Crafoord Foundation, Sweden. We acknowledge the Institut Laue-Langevin for the awarded beamtime (8-02-902, 10.5291/ILL-DATA.8-02-902) and for providing access to the PSCM laboratories, and we thank the European Synchrotron Radiation Facility (ESRF) for assisting in using beamline BM29 (MX2387, 10.15151/ESRF- ES-515918997; MX2508, 10.15151/ESRF-ES-1026511637).

Contact

Prof. Marie Skepö

Lund University

marie.skepo@compchem.lu.se