Self-assembly and adhesive properties of <i>Pollicipes pollicipes</i> barnacle cement protein cp19k: influence of pH and ionic strength

Marine organisms such as barnacles rely on a complex underwater adhesive system, driven by self-assembly and intermolecular associations between cement proteins, for permanent attachment to a variety of surface types. In this study, we investigated the influence of environmental parameters on the self-assembly of recombinant cp19k, a key adhesive protein in <i>Pollicipes pollicipes</i>. Using TEM imaging, a low pH (4.0) and high salt concentration (600 mM NaCl) environment, mimicking <i>P. pollicipes</i> gland conditions, was identified to promote the formation of extended, needle-like fibrils by the cp19k protein. The β-amyloid nature of fibrils formed under these conditions and at high pH/low salt concentration was confirmed by Thioflavin T assay. Non-fibrillar cp19k adhered most effectively to hydrophilic and hydrophobic surfaces under low pH/low salt concentration conditions, while pre-formed fibrils retained their adhesion ability upon switching to a high pH/high salt concentration environment, which was designed to mimic the change in the protein environment upon secretion in vivo. These findings support the hypothesis that fibril formation occurs in the acidic, iso-osmotic gland of the barnacle, with delayed cement curing enabling fibril secretion for sustained adhesion of the organism. The study provides insight into the environmental sensitivity of cp19k structure-function dynamics and may support the design of bioinspired adhesives and biomaterials.