Very Stable High-Molecular-Mass Multiprotein Complexes in Different Organs of the Sea Cucumber Paracaudina chilensis

We report the first identification of several large (1.4-2.2 MDa), highly stable protein-peptide complexes in various organs and tissues (body wall, gonads, respiratory trees, gut, and coelomic fluid) of the sea cucumber Paracaudina chilensis. Gel filtration and transmission electron microscopy methods were used to estimate the molecular weights and sizes of the complexes. According to light scattering assay data, these multiprotein complexes undergo significant dissociation only in the presence of 3.0 M MgCl₂ or 8.0 M urea containing 0.1 M EDTA and DTT. Analysis of the complexes using SDS-PAGE and MALDI mass spectrometry showed that all complexes contain numerous proteins (>10 kDa), whose number and composition vary among organs. Additionally, using MALDI mass spectrometry, it was shown that the whole-organism complexes contain 254 distinct peptides (<10 kDa). The peptide content in organ-specific complexes decreases in the following order: respiratory trees (104) > coelomic fluid (76) > body wall (64) > gut (58) > gonads (55). In contrast to individual proteins and peptides, multiprotein complexes have expanded possibilities, since they can interact with various molecules and cells. Thus, they can perform the functions of all peptides and proteins located on their surfaces. We propose that the unique protein and peptide composition of each complex facilitates the specific biological functions of its respective organ.