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A number of fully functional proteins have been identified to exist in a partially or fully disordered state. These intrinsically disordered proteins (IDP) are recognized as an important sequence class that fulfill many roles. A number of biomineral-associated proteins, particularly those which possess polyelectrolyte domains, represent potential members of the IDP class. This report describes a bioinformatics study of a ten member polyanionic sequence biomineralization protein family, Asprich, and the experimental characterization of the conserved N- and C-terminal regions found within seven members of this family. Using protein disorder prediction algorithms (DPROT, PONDR, GLOBPLOT), we confirm that all ten Asprich protein sequences are disordered, and that two polyelectrolyte domains within each protein contribute to the disorder scoring. Using synthetic peptides which model the conserved N- (F1,48 AA) and C-terminal (F2, 42 AA) domains, we determine that both domains are globally disordered and remain so in the presence of Ca(II). However, F1 and F2 possess differing proportions of extended beta strand relative to random coil structure and sequence spacing of Asp, Glu residues. As a result, the F2 sequence possesses a higher anionic surface charge density, solvent accessibility, and greater degree of local conformational response to Ca(II). These differences may explain why F1 and F2 differ with regard to step growth kinetics, mineral modulation, and metal ion complexation, and possibly distinguish the molecular role(s) that each domain conveys to the Asprich protein family. Structural and surface charge density features may also control the function of Asp, Glu polyelectrolyte domains within other IDP proteins.

The RING or Really Interesting New Gene represents a family of eukaryotic sequences that bind Zn (II) ions and participate in intracellular processes involving protein-protein interaction. Although found in over 400 different proteins, very little is known regarding the structure-function properties of these domains because of the aggregation problems associated with RING sequences. To augment this data set, we report an unusual 36 AA C-terminal sequence of an extracellular matrix mollusk shell protein, AP7, that exhibits partial homology to the RING family. This Cys, His-containing sequence, termed AP7C, binds Zn (II) and other multivalent ions, but does not utilize a tetracoordinate complexation scheme for binding such as that found in Zn (II) finger polypeptides. Moreover, unlike Zn (II) finger and RING domains, this 36 AA can fold into a relatively stable structure in the absence of Zn (II). This folded structure consists of three short helical segments (A, B, and C), with segments A and B separated by a 4 AA type I beta-turn region and segments B and C separated by a 7 AA loop-like region. Interestingly, the putative RING-like region, -RRPFHECALCYSI-, experiences slow conformational exchange between two structural states in solution, most likely in response to imido ring interconversion at P8 and P21. Poisson-Boltzmann solvation calculations reveal that the AP7C molecular surface possesses a cationic region near its N-terminus, which lies adjacent to the 30 AA mineral modification domain in the AP7 protein. Given that the AP7C sequence does not influence mineralization, it is probable that this cationic pseudo-RING region is utilized by the AP7 protein for other tasks such as protein-protein interaction within the mollusk shell matrix.