Evidence of Mineralization Activity and Supramolecular Assembly by the N-Terminal Sequence of ACCBP, a Biomineralization Protein That Is Homologous to the Acetylcholine Binding Protein Family

Several biomineralization proteins that exhibit intrinsic disorder also possess sequence regions that are homologous to nonmineral associated folded proteins. One such protein is the amorphous calcium carbonate binding protein (ACCBP), one of several proteins that regulate the formation of the oyster shell and exhibit 30% conserved sequence identity to the acetylcholine binding protein sequences. To gain a better understanding of the ACCBP protein, we utilized bioinformatic approaches to identify the location of disordered and folded regions within this protein. In addition, we synthesized it 50 AA polypeptide, ACCN, representing the N-terminal domain of the mature processed ACCBP protein. We then utilized this polypeptide to determine the mineralization activity and qualitative structure of the N-terminal region of ACCBP. Our bioinformatic Studies indicate that ACCBP consists of a ten-stranded beta-sandwich structure that includes short disordered sequence blocks, two of which reside within the primarily helical and surface-accessible ACCN sequence Circular dichroism Studies reveal that ACCN is partially disordered in Solution, however, ACCN can be Induced to fold into an alpha helix in the presence of TFE Furthermore, we confirm that the ACCN sequence IS Multifunctional: this sequence promotes radial calcite polycrystal growth oil Kevlar threads and forms supramolecular assemblies in solution that contain amorphous-appearing deposits. We conclude that the partially disordered ACCN sequence is a putative site for mineralization within the ACCBP protein and that the presence of short disordered sequence regions within the ACCBP activity fold are essential for function.