Aspein is an unusually acidic

bivalve shell A bivalve shell is the enveloping exoskeleton or mollusc shell, shell of a bivalve mollusc, composed of two hinged halves or ''valve (mollusc), valves''. The two half-shells, called the "right valve" and "left valve", are joined by a ligament and ...

matrix

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

, which may have important roles in calcium carbonate

biomineralization Biomineralization, also written biomineralisation, is the process by which living organisms produce minerals, often resulting in hardened or stiffened '' mineralized tissues''. It is an extremely widespread phenomenon: all six taxonomic kingd ...

. The Aspein gene (''aspein'',

Uniprot UniProt is a freely accessible database of protein sequence and functional information, many entries being derived from genome sequencing projects. It contains a large amount of information about the biological function of proteins derived fro ...

Q76K52
encodes a sequence of 413

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

s, including a high proportion of Asp (60.4%), Gly (16.0%), and Ser (13.2%), and the predicted

isoelectric point The isoelectric point (pI, pH(I), IEP), is the pH at which a molecule carries no net electric charge, electrical charge or is electrically neutral in the statistical mean. The standard nomenclature to represent the isoelectric point is pH(I). Howe ...

is 1.78. This is the most acidic of all the molluscan shell matrix proteins sequenced so far, or probably even of all known proteins on earth. The main body of aspein is occupied by (Asp)2–10 sequences punctuated with Ser–Gly

dipeptides A dipeptide is an organic compound derived from two amino acids. The constituent amino acids can be the same or different. When different, two isomers of the dipeptide are possible, depending on the sequence. Several dipeptides are physiological ...

RT-PCR Reverse transcription polymerase chain reaction (RT-PCR) is a laboratory technique combining reverse transcription of RNA into DNA (in this context called complementary DNA or cDNA) and amplification of specific DNA targets using polymerase chain ...

demonstrated that the transcript of aspein is expressed at the outer edge of the mantle, corresponding to the calcitic

prismatic An optical prism is a transparent optics, optical element with flat, polished surfaces that are designed to refraction, refract light. At least one surface must be angled—elements with two parallel surfaces are ''not'' prisms. The most fami ...

layer, but not at the inner part of the mantle, corresponding to the aragonitic nacreous layer. Aspein is likely responsible for directed formation of

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

in the shell of the pearl oyster '' Pinctada fucata''.

References

{{Reflist Extracellular matrix proteins Mollusc shells