Abstract
The extensible byssus is produced by the foot of bivalve animals, including the pearl oyster Pinctada fucata, and enables them to attach to hard underwater surfaces. However, the mechanism of their extensibility is not well understood. To understand this mechanism, we analyzed the ultrastructure, composition and mechanical properties of the P. fucata byssus using electron microscopy, elemental analysis, proteomics and mechanical testing. In contrast to the microstructures of Mytilus sp. byssus, the P. fucata byssus has an exterior cuticle without granules and an inner core with nanocavities. The removal of Ca2+ by ethylenediaminetetraacetic acid (EDTA) treatment expands the nanocavities and reduces the extensibility of the byssus, which is accompanied by a decrease in the β-sheet conformation of byssal proteins. Through proteomic methods, several proteins with antioxidant and anti-corrosive properties were identified as the main components of the distal byssus regions. Specifically, a protein containing thrombospondin-1 (TSP-1), which is highly expressed in the foot, is hypothesized to be responsible for byssus extensibility. Together, our findings demonstrate the importance of inorganic ions and multiple proteins for bivalve byssus extension, which could guide the future design of biomaterials for use in seawater.
Highlights
P. fucata anchor themselves onto almost any surface, including glass, rock, plastic or other shells, through the byssus (Fig. 1a), which typically consists of 3–10 threads 1–2 cm in length (Fig. 1b)
We demonstrate the critical roles of Ca2+-stabilized nanocavities and TSP-1 proteins for byssus extension in Pinctada fucata
Byssus extensibility is hypothesized to be related to the interaction of byssal proteins with Ca2+ in the distal region of the byssus, which is composed of cuticle and a core containing nanocavities
Summary
The elastic proteins in the byssus of the giant clam (Tridacna maxima) form a stable rope-like structure containing tetrameric coiled-coil α -helices[14]. We showed the importance of inorganic ions and byssal proteins for byssus extensibility in the bivalvia pearl oyster Pinctada fucata. Through ultrathin sectioning and electron microscopy, we showed that the cuticle of byssus is composed of compact protein fibers, whereas the core is composed of loose protein nanofibers with nanocavities. Fourier transform infrared spectroscopy (FTIR) and mechanical tests were performed to investigate the effect of Ca2+ on byssal protein conformation and the mechanical properties of the byssus, respectively. A thrombospondin-1 (TSP-1) is proposed to be accountable for the extensibility of the byssus based on sequence analysis and real-time PCR
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