Presence of structural homologs of ubiquitin in haloalkaliphilic Archaea

Ubiquitin, a protein widely conserved in eukaryotes, is involved in many cellular processes, including proteolysis. While sequences encoding ubiquitin-like proteins have not been identified in prokaryotic genomes sequenced so far, they have revealed the presence of structural and functional homologs of ubiquitin in Bacteria and Archaea. This work describes the amplification and proteomic analysis of a 400-bp DNA fragment from the haloalkaliphilic archaeon Natrialba magadii. The encoded polypeptide, P400, displayed structural homology to ubiquitin-like proteins such as those of the ThiS family and Urm1. Expression of the P400 DNA sequence in Escherichia coli cells yielded a recombinant polypeptide that reacted with anti-ubiquitin antibodies. In addition, a putative open reading frame encoding P400 was identified in the recently sequenced genome of N. magadii. Together, these results evidence the presence in Archaea of structural homologs of ubiquitin- related proteins.