CLONING OF THE POLYUBIQUITIN CDNA FROM THE MARINE SPONGE GEODIA-CYDONIUM AND ITS PREFERENTIAL EXPRESSION DURING REAGGREGATION OF CELLS

Ubiquitination of proteins is a critical step in the controlled degrad ation process of many polypeptides. Here we show that sponges, the sim plest multicellular group of eukaryotic organisms, are also equipped w ith the ubiquitin pathway. The polyubiquitin cDNA was isolated and cha racterized from the marine sponge Geodia cydonium. The open reading fr ame contains six ubiquitin moieties, which are lined up head to tail w ithout spacers. A comparison of the predicted amino acid sequence of t he six sponge ubiquitin-coding units with those from other organisms r evealed a high degree of homology (> 93%). The ubiquitin gene is expre ssed to almost the same extent in the two main compartments of the spo nge, the cortex and the medulla. However, only in the cortex are detec table amounts of the ubiquitin protein synthesized. The ubiquitin prot ein isolated from the sponge organism was found to initiate protein de gradation in the heterologous reticulocyte system in the same manner a s bovine ubiquitin. In vitro studies with dissociated sponge cells rev ealed that the homologous aggregation factor causes (i) a strong incre ase in the steady-state level of mRNA coding for ubiquitin and (ii) a drastic increase in ubiquitin protein synthesis, while the homologous lectin failed to display that effect in isolated cells. These data sug gest that ubiquitin may play a role in sponge morphogenesis.