Unique proteins from the statoliths of Lolliguncula brevis (Cephalopoda: Loliginidae)

Information concerning the role of the organic matrix (OM) in statolith mineralization may contribute to resolving problems currently facing the use of increments in squid statoliths to estimate the age of individuals. A preliminary study aimed at purifying and characterizing the OM proteins from statoliths of the loliginid squid Lolliguncula brevis is described. Proteins extracted from the statoliths were separated into two fractions, insoluble and soluble in aqueous solutions. Gel electrophoresis indicated that the insoluble fraction is composed of at least eight major proteins, ranging in size from 25 to over 200 kDa. The five largest insoluble proteins were glycosylated, as shown by immunoassay. In contrast, the soluble fraction is composed of a single dominant protein of about 100 kDa, and two other major proteins of higher molecular weight. All three soluble proteins were glycosylated. Molecular weight and partial sequence data of peptides from five of the insoluble and one of the soluble proteins were used to search databases for possible homologs. No matches were found, suggesting that these proteins may belong to a class of hitherto undescribed OM compounds. Different proteins are involved in biomineralization processes in different organisms. We propose that the insoluble matrix proteins are responsible for regulating the appearance of increments in squid statoliths.