It is clear that nacre- and prismatic-associated terminal sequences exhibit important similarities and differences (Tables 1 and 2). The major similarity for both classes is structural. Prior to their interaction with mineral surfaces and/or ionic and water clusters, we would predict that the majority of the nacre and prismatic terminal sequences will exist as unfolded, conformationally labile species in solution. However, protein - mineral and protein - cluster interaction processes would initiate structural reordering to some degree, and then conformationally unstable regions of each sequence would become stabilized. This hypothesis needs to be experimentally tested, and it is hoped that in the near future methods and suitable mollusk shell biomineralization protein candidates will become available for this purpose. What appears to distinguish prismatic from nacre sequences are two important features: electrostatics and the usage of Pro, Gly, and Cys. We will discuss electrostatics first. In general, the nacre sequences possess a heterogeneously distributed mixture of anionic and cationic residues that gives rise to either a net (+) or (-) charge on the corresponding polypeptide (Table 1). Thus, compared to prismatic sequences, nacre terminal sequences will possess molecular surfaces with a heterogeneous charge distribution of (+) and (-). In comparison, the prismatic sequence library possesses multiple anionic residues and net anionic charge. When cationic residues do occur in these prismatic sequences, they tend to be segregated from anionic residues. As a consequence, prismatic terminal sequences have a greater degree of (+), (-) charge separation and the resulting molecular surfaces will reflect this. These differences in charge distribution will have an impact on polypeptide - mineral surface recognition and possibly define the potential sites on the mineral surface where nacre or prismatic terminal sequences preferentially interact. For similar reasons, ionic clustering on polypeptide molecular surfaces would also be expected to be different for nacre and prismatic terminal sequences. In particular, those prismatic terminal sequences which are polyelectrolyte in nature will foster significant counterion condensation at their molecular surfaces and offer homogeneous multiple sites for mineral surface or ion interactions, compared to their nacre counterparts. Collectively, these traits can differentially "tune" the recognition, interaction, and specificity of nacre and prismatic sequence binding to mineral surfaces and influence the stoichiometry (i.e., Ca(II)/CO2 3-) of bound ions on each type of sequence class. Let us now consider the occurrence of Cys, Pro, and Gly in both terminal sequence ensembles and their potential impact. Compared to prismatic-specific sequences, nacre-specific terminal sequences utilize Cys residues to a greater extent. Although the significance of Cys is not completely understood, reports of internal disulfide linkages and their polypeptide folding implications17,18 suggest that Cys may play an important role in defining the conformation, stability, and function of nacre-specific terminal sequences. Conversely, the fact that prismatic sequences are deficient in Cys suggests that disulfide linkages are rare within the prismatic ensemble and/or that Cys is not a significant requirement for either the functional or structural aspects of these sequences. Gly and Pro are found in both nacre and prismatic terminal sequences, suggesting that there is a need to "tune" the molecular motion or conformation within both types of sequences, as well as introduce loop- (Gly)47-49 or PPII-like (Pro, Ala, Gln)50-52,56-60 structural regions for functionality. With the exception of the prismatic-associated MSP1,2 protein family, there appears to be a higher occurrence of Gly and/or Pro in the nacre sequence ensemble, which would indicate that nacre terminal sequences have a greater reliance on these residues for functional purposes. It is hoped that this review of mollusk shell terminal sequences will encourage further scientific investigations of the structural and functional aspects of these sequences and the parent proteins from which they are derived. Obviously, nonterminal sequence regions have been excluded from consideration in this review, but it would be foolish to suggest that these regions do not play a role in the mollusk shell mineralization process on some level. Over time, we hope that comparative sequence studies and structure/function investigations will be conducted on these other sequence regions to provide a more comprehensive view.
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