A novel 19 kDa protein, which was named N19, was isolated from the nacreous layer of the pearl oyster Pinetada fucata. N19 is one of predominant proteins found in the water-insoluble fraction of the nacreous layer. MALDI-TOF/TOF analysis indicated that the three trypsin-digested peptides (791.45, 824.42, and 1118.65 m/z) corresponded to the amino acid sequences predicted from a cDNA isolated from a mantle cDNA library of P. fucata. Northern blot analysis revealed that the N19 mRNA was a little more abundant in the pallial region than the edge region, in the mantle. In CaCO3 precipitation assay, the recombinant N19 protein inhibited the crystallization of CaCO3. These results indicate that N19 is localized in the nacre and plays a negative regulatory role in calcification in the pearl oyster. (C) 2007 Elsevier Inc. All rights reserved.