Mechanical and Biophysical Characterization of Structural Low-Complexity Marine Proteins
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Mechanical and Biophysical Characterization of Structural Low-Complexity Marine Proteins


Natural materials are often endowed with enviable properties and understanding the mechanisms from which these properties originate yields both fascinating fundamental science and crucial information for successful bioinspired engineering. Protein building blocks of natural materials are a principal source of bioinspiration and can be understood as sequence- specific polymers that come together to form complex 3-dimensional structures in order to serve a vast array of functions. Although the complexity of sequence and structure in typical folded proteins make bioinspired design challenging, there are examples of highly functional proteins whose sequences are extremely biased or repetitive, which may lead to simpler translation synthetically. This work investigates the biochemical, molecular, and mechanical properties of proteins from three such load-bearing materials from marine organisms. In the first example from mussel holdfasts, the mechanical effects of the protein backbone are investigated by comparing the adhesive response of a series of low-complexity mussel-derived peptides with their poly-N-substituted glycine analogs, otherwise known as peptoids. The peptoids exhibit strikingly different surface deposition behavior, as they form dehydrated monolayers rather than the hydrated multilayers formed by polypeptides. There is also divergent adhesive behavior, which we propose arises primarily from the differences in film hydration. In the second, the major polymeric component of the lightweight, melanized, and wear-resistant jaws of Glycera dibranchiata is identified as a 22kDa protein dominated by two amino acids: 50% glycine and 30% histidine. We name it MultiTasking Protein (MTP) for the number of functions it displays in vitro. MTP shows high capacity Cu2+ binding, which in turn, triggers liquid-liquid phase separation. Phase separation also leads to the formation of viscous film at the air-water interface. We find that when this process co-occurs with melanin biogenesis, the films become more elastic than viscous, hence provide some insight into how Glycera jaws are formed in situ. Third, the hitherto overlooked protective eggcase threads of Octopus bimaculoides are subjected to biochemical and mechanical analyses. The threads have a fibrous internal structure and are primarily composed of a single cystine-rich protein. This protein, which we name octovafibrin (octopus ovarian fibrous protein), is characterized by C-type lectin binding domains at the N- and C-termini, with core of 27 repeat epidermal growth factor (EGF)-like domains. Tensile testing reveals behavior akin to semicrystalline elastomers, and the experiments indicate the mechanical response is mediated by fiber rearrJangement as well as distortion of the EGF-like domains. Each study provides the groundwork for further inquiry and the basis for future low-complexity bioinspired synthetic mimics.

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