Molecular mechanism of the nacreous layer formation in Pinctada maxima

We have cloned the cDNAs that encode two kinds of molluscan shell matrix pr oteins, namely N66 and N14, in the nacreous layer of Pinctada maxima. N66 i s composed of carbonic anhydrase-like and repeat domains, as described for nacrein (1) in the pearls of P. fucata. N14 is homologous to N16, recently found in the nacreous layer of P. fucata (2) and is characterized by high p roportions of Gly, Tyr, and Asn together with NG repeat sequences. The mole cular weights of these proteins were estimated as 59,814 and 13,734 Da, res pectively. Structural differences were clearly indicated in the alignment a nd length of the repeat sequences of the sets of the homogeneous proteins ( NBB/nacrein and N14/N16). The longer repeat sequences of N66 and N14 may be responsible for P. maxima's excellent property of calcification. The in vi tro crystallization experiments revealed that the mixture of N66 and N14 co uld induce platy aragonite layers highly similar to the nacreous layer, onc e adsorbed onto the membrane of the water-insoluble matrix. (C) 2000 Academ ic Press.