Proteins of morula-like cells in hemolymph of the giant clam, Tridacna derasa.

Abstract

The morula-like cell, a hemocyte packed with many large (about 3 microns in diameter) electron-dense granules, is found only in the hemolymph of giant clams belonging to the Tridacnidae. To clarify the function of the morula-like cell, we investigated its proteins, especially those found in the large granules. Proteins with molecular weights of 64 kDa, 17 kDa and 7.4 kDa were found to be specific to this type of hemocyte. N-terminal amino acid sequence analysis revealed that the 17-kDa and 7.4-kDa proteins were novel proteins rich in aromatic amino acids. Rabbit polyclonal antibody against a synthetic peptide of the 7.4-kDa protein reacted not only with that protein but also with a larger molecular weight (about 16-kDa) protein in the morula-like cell. Examination of the N-terminal amino acid sequences showed that the 16-kDa protein is distinct from the 17-kDa protein, and Western blot analysis suggested that it is a precursor of the 7.4-kDa protein. The zooxanthellate portion of clam mantle and kidney contained proteins immunoreactive to the antibody, but the azooxanthellate portion of the mantle did not contain any immunoreactive protein. These results suggest that the morula-like cells interact with the zooxanthellae.