MOLECULAR-CLONING AND CHARACTERIZATION OF LUSTRIN-A, A MATRIX PROTEINFROM SHELL AND PEARL NACRE OF HALIOTIS-RUFESCENS

A specialized extracellular matrix of proteins and polysaccharides con trols the morphology and packing of calcium carbonate crystals and bec omes occluded within the mineralized composite during formation of the molluscan shell and pearl. We have cloned and characterized the cDNA coding for Lustrin A, a newly described matrix protein from the nacreo us layer of the shell and pearl produced by the abalone, Haliotis rufe scens, a marine gastropod mollusc. The full-length cDNA is 4,439 base pairs (bp) long and contains an open reading frame coding for 1,428 am ino acids. The deduced amino acid sequence reveals a highly modular st ructure with a high proportion of Ser (16%), Pro (14%), Gly (13%), and Cys (9%). The protein contains ten highly conserved cysteine-rich dom ains interspersed by eight proline-rich domains; a glycine-and serine- rich domain lies between the two cysteine-rich domains nearest the C t erminus, and these are followed by a basic domain and a C-terminal dom ain that is highly similar to known protease inhibitors. The glycine-a nd serine-rich domain and at least one of the proline-rich domains sho w sequence similarity to proteins of two extracellular matrix superfam ilies (one of which also is involved in the mineralized matrixes of bo ne, dentin, and avian eggshell). The arrangement of alternating cystei ne-rich domains and proline-rich domains is strikingly similar to that found in frustulins, the proteins that are integral to the silicified cell wall of diatoms. Its modular structure suggests that Lustrin A i s a multifunctional protein, whereas the occurrence of related sequenc es suggest it is a member of a multiprotein family.