6533b874fe1ef96bd12d6061

RESEARCH PRODUCT

Ubiquitins (polyubiquitin and ubiquitin extension protein) in marine sponges: cDNA sequence and phylogenetic analysis

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description

The complete nucleotide sequences of twoSuberites domunculacDNAs and oneSycon raphanuscDNA, all encoding ubiquitin, have been determined. One cDNA fromS. domunculacodes for polyubiquitin with four tandemly repeated monomeric units and the second cDNA encodes ubiquitin fused to a ribosomal protein of 78 amino acids (aa).S. domunculapossesses at least one additional polyubiquitin gene, from which the last two monomers were also sequenced. All analysed genes fromS. domunculaencode identical ubiquitin proteins, with only one aa difference (Ala19) to the human/higher animals ubiquitin (Pro19). Ubiquitin inS. domunculais identical with the ubiquitin found in another Demospongia,Geodia cydonium. The cDNA fromS. raphanusencodes polyubiquitin with seven tandemly repeated units. All these gene monomers code for the same ubiquitin, which differs from the human/higher animals ubiquitin only at position 24 (Asp inSycon, Glu in others). However, ubiquitin fromS. raphanus(Calcarea) shows two aa differences (positions 19 and 24), when compared with the ubiquitin sequences from the two Demospongiae. In a phylogenetic tree constructed by multiple sequence alignment of all sponge ubiquitin gene monomers so far identified, all monomers from the same species cluster together, with the clear exception of the monomer fromS. domuncularibosomal protein fusion gene. This monomer branches off first from the tree and forms a separate line; this gives evidence for a very ancient split of ubiquitin-ribosomal-protein fusion genes from polyubiquitin encoding genes and their long separate coexistence in eukaryotes. The ubiquitin extension protein fromS. domunculais 78 aa long, displays all characteristics of 76–81 aa long ribosomal fusion proteins and shows 78% identity in the first 73 aa with the human S27a protein. However, its C-terminal sequence: 69-GLTYVYKKSD-78 is more similar to the plant consensus (69-GLTYVYQ/NK-76), than to the higher animal consensus (69-CLTYCFNK-76). This protein isolated from a sponge, belonging to the phylogenetically oldest multicellular animals, the Porifera, branches off first from the phylogenetic tree of metazoan ubiquitin extension proteins of the small ribosomal subunits.