Ubiquitin and ubiquitination in cells from the marine sponge Geodia cydonium

Marine sponges, e.g. Geodia cydonium, have been intensively used to investigate the biochemical and molecular biological basis of cell-cell- and cell-matrix adhesion. It has been shown that a family of galactose-specific lectins, which are present in the extracellular space of G. cydonium, is a main component involved in cell-matrix adhesion in the sponge system. In the present study it is outlined that the purified 16-kDa lectin-1 binds to a 67-kDa membrane-associated protein. This lectin-binding protein undergoes mono- and diubiquitination after incubation of dissociated sponge cells with the homologous aggregation factor (AF), a molecule involved in cell-cell adhesion. The gene coding for polyubiquitin was characterized and found to be composed of three tandem repeat building blocks. Northern analysis indicated the presence of only one type of ubiquitin-specific mRNA (1.65 kb). The level of this transcript increased by 10-fold after incubation of the dissociated cells with AF for 8 h; in contrast, lectin-1 caused only a small effect on the steady-state level of ubiquitin mRNA. These data indicate that the expression of the polyubiquitin gene is directly or indirectly regulated by the AF and suggest that ubiquitination might be a process which controls the function of the membrane-associated lectin-binding protein during matrix-cell adhesion.