Nmag_2608, an extracellular ubiquitin-like domain-containing protein from the haloalkaliphilic archaeon Natrialba magadii

Ubiquitin-like proteins (Ubls) and ubiquitin-like domain-containing proteins (Ulds) found in both eukaryotes and prokaryotes display an ubiquitin fold. We previously characterized a 124-amino acid polypeptide (P400) from the haloalkaliphilic archaeon having structural homology with ubiquitin family proteins. The reported 's genome allowed the identification of the gene for the protein containing P400, which belongs to specific orthologs of halophilic organisms. It was found that Nmag_2608 has an N-terminal signal peptide with a lipobox motif characteristic of bacterial lipoproteins. Also, it presents partial identity with the ubiquitin-like domain-containing proteins, soluble ligand binding beta-grasp proteins. Western blots and heterologous expression tests in evidenced that Nmag_2608 is processed and secreted outside the cell, where it could perform its function. The analysis of expression in 's cells suggests a co-transcription with the adjoining gene encoding a protein of the cyclase family. Also, the transcript level decreased in cells grown in low salinity and starved. To conclude, this work reports for the first time an extracellular archaeal protein with an ubiquitin-like domain.