Molecular mechanism of the nacreous layer formation in Pinctada maxima

We have cloned the cDNAs that encode two kinds of molluscan shell matrix proteins, namely N66 and N14, in the nacreous layer of Pinctada maxima. N66 is 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 proportions of Gly, Tyr, and Asn together with NG repeat sequences. The molecular weights of these proteins were estimated as 59,814 and 13,734 Da, respectively. Structural differences were clearly indicated in the alignment and length of the repeat sequences of the sets of the homogeneous proteins (N66/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 vitro crystallization experiments revealed that the mixture of N66 and N14 could induce platy aragonite layers highly similar to the nacreous layer, once adsorbed onto the membrane of the water-insoluble matrix.