The lustrin superfamily represents a unique group of biomineralization proteins localized between layered aragonite mineral plates (i.e., nacre layers) in mollusk shell. These proteins exhibit elastomeric and adhesive behavior within the mineralized matrix. One member of the lustrin superfamily, Lustrin A, has been sequenced; the protein is organized into defined, modular sequence domains that are hypothesized to perform separate functions (e.g., force unfolding, mineral interaction, intermolecular binding) within the Lustrin A protein. Using NMR, CD spectrometry, and model peptides, we investigated the solution structure of two Lustrin A polyelectrolyte modular domains that represent putative sites for Lustrin A-nacre component interaction: the 30-AA Arg, Lys, Tyr, Ser-rich (RKSY), and 24-AA Asp-rich (D4) domains. The results indicate that both sequences adopt open unfolded structures, with RKSY exhibiting structural features of an extended conformer, and D4 a more labile, random-coil conformation. These results suggest that the Lustrin A protein possesses open, unfolded regions that could act as putative sites for Lustrin A-mineral or Lustrin A-macromolecular interactions that lead to the observed adhesive properties of the nacre organic matrix.