In
the sea urchin embryo spicule, there exists a proteome of >200
proteins that are responsible for controlling the mineralization of
the spicule and the formation of a fracture-resistant composite. In
this report, using recombinant proteins, we identify that two protein
components of the spicule, SM30B/C and SM50, are hydrogelators. Because
of the presence of intrinsic disorder and aggregation-prone regions,
these proteins assemble to form porous mesoscale hydrogel particles
in solution. These hydrogel particles change their size, organization,
and internal structure in response to pH and ions, particularly Ca(II),
which indicates that these behave as ion-responsive or “smart”
hydrogels. Using diffusion-ordered spectroscopy NMR, we find that
both hydrogels affect the diffusion of water, but only SM50 affects
the diffusion of an anionic solute. Thus, the extracellular matrix
of the spicule consists of several hydrogelator proteins which are
responsive to solution conditions and can control the diffusion of
water and solutes, and these proteins will serve as a model system
for designing ion-responsive, composite, and smart hydrogels.
In the mollusk shell there exists
a framework silk fibroin–polysaccharide
hydrogel coating around nacre aragonite tablets, and this coating
facilitates the synthesis and organization of mineral nanoparticles
into mesocrystals. In this report, we identify that a protein component
of this coating, n16.3, is a hydrogelator. Due to the presence of
intrinsic disorder, aggregation-prone regions, and nearly equal balance
of anionic and cationic side chains, this protein assembles to form
porous mesoscale hydrogel particles in solution and on mica surfaces.
These hydrogel particles change their dimensionality, organization,
and internal structure in response to pH and ions, particularly Ca(II),
which indicates that these behave as ion-responsive or “smart”
hydrogels. Thus, in addition to silk fibroins, the gel phase of the
mollusk shell nacre framework layer may actually consist of several
framework hydrogelator proteins, such as n16.3, which can promote
mineral nanoparticle organization and assembly during the nacre biomineralization
process and also serve as a model system for designing ion-responsive,
composite, and smart hydrogels.
We examined the mineralization performance of a nacre protein, AP7, within seawater mineralization assays that form aragonite and magnesium calcite. Under these conditions AP7 forms hydrogel particles that vary in size and complexity depending upon ionic conditions. These hydrogels "hijack" the mineralization process by limiting nucleation in bulk solution and promoting nucleation within the hydrogels.
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