Two unique membrane-bound AAA proteins from Sulfolobus solfataricus.

Thermoacidophilic crenarchaea of the genus Sulfolobus contain six AAA (ATPase associated with various cellular activities) proteins, including a proteasome-associated ATPase, a Vps4 (vacuolar protein sorting 4) homologue, and two Cdc48 (cell-division cycle 48)-like proteins. The last two AAA proteins are deeply branching divergent members of this family without close relatives outside the Sulfolobales. Both proteins have two nucleotide-binding domains and, unlike other members of the family, they seem to lack folded N-terminal domains. Instead, they contain N-terminal extensions of approx. 50 residues, which are predicted to be unstructured, except for a single transmembrane helix. We have analysed the two proteins, MBA (membrane-bound AAA) 1 and MBA2, by computational and experimental means. They appear to be monophyletic and to share a common ancestor with the Cdc48 clade. Both are membrane-bound and active as nucleotidases upon heterologous expression in Escherichia coli. They form ring complexes, which are stable after solubilization in a mild detergent and whose formation is dependent on the presence of the N-terminal extensions.