FtsH recognizes proteins with unfolded structure and hydrolyzes the carboxyl side of hydrophobic residues

FtsH of Escherichia coli is an essential membrane-integrated ATP-dependent protease. We cloned a gene for an FtsH homolog (T.FtsH) from Thermus thermophilus HB8, expressed it in E. coli, and purified the expressed protein. ATPase activity of T.FtsH was activated by proteins with unfolded structure (alpha-casein and pepsin), and T.FtsH digested these proteins in an ATP-, Zn2+-dependent manner. alpha-Lactalbumin was digested by T.FtsH when it was largely unfolded, but not in its native form. Analysis of the proteolytic products revealed that, in most cases, T.FtsH cleaved the C-terminal side of hydrophobic residues and produced a characteristic set of small peptides (<30 kDa) without releasing a large intermediate. Thus, T.FtsH recognizes the unfolded structure of the proteins and progressively digests them at the expense of ATP. A soluble domain of T.FtsH, which lacked the N-terminal two transmembrane helices, was also prepared but was found to retain neither ATPase nor protease activities. Thus, the membrane segment appeared to be indispensable for these activities of T.FtsH.