Roles of multimerization and membrane association in the proteolytic functions of FtsH (HflB)

FtsH (HflB) is an Escherichia coli ATP-dependent protease that degrades some integral membrane and cytoplasmic proteins. While anchored to the cytoplasmic membrane by the two transmembrane (TM) segments near the N-terminus, it has a large cytoplasmic domain. The N-terminal region also has a role in homo-oligomerization of this protein. To study the significance of the membrane integration and oligomer formation, we constructed FtsH derivatives in which the N-terminal region had been deleted or replaced with either the leucine zipper sequence from Saccharomyces cerevisiae GCN4 protein or TM regions from other membrane proteins. The cytoplasmic domain, which was monomeric and virtually inactive, was converted, by the attachment of the leucine zipper, to an oligomer with proteolytic function against a soluble, but not a membrane-bound substrate. In contrast, chimeric TM-FtsH proteins were active against both substrate classes. We suggest that the cytoplasmic domain has intrinsic but weak self-interaction ability, which becomes effective with the aid of the leucine zipper or membrane tethering, and that membrane association is essential for FtsH to degrade integral membrane proteins.