Product of a new gene, syd, functionally interacts with SecY when overproduced in Escherichia coli

FtsH (HflB) is an ATP-dependent protease selectively acting on SecY and some other membrane proteins

The FtsH protein is a membrane-bound ATPase of Escherichia coli that was proposed to be involved in membrane protein assembly as well as degradation of some unstable proteins. SecY, a subunit of protein translocase, is FtsH dependently degraded in vivo when it fails to associate with its partner (the SecE protein). We constructed a series of mutants in which mutations were introduced into conserved residues in the two ATP binding consensus sequences or the zinc binding sequence of FtsH. We purified wild-type and mutant FtsH proteins by making use of a polyhistidine tag attached to their carboxyl termini. Complementation analysis and ATPase activity assays in vitro indicated that, of the two sets of ATP binding sequence motifs, the one located C-terminally (A1) is essential for ATPase activity and in vivo functioning of FtsH. Wild-type FtsH protein degraded purified SecY in an ATP hydrolysis-dependent manner in vitro. Mutant proteins without ATPase activity were inactive in proteolysis. A zinc binding motif mutant showed a decreased proteolytic activity. SecY and FtsH were cross-linkable with each other in the membrane, provided that FtsH had an ATPase-inactivating mutation. These results demonstrate that FtsH binds to and degrades SecY, its A1 motif and the zinc binding motif being important for the proteolytic activity. FtsH-dependent proteolysis was also demonstrated for SecY in crude membrane extracts, whereas a majority of other membrane proteins were not degraded, indicating that FtsH has high selectivity in protein degradation.

Pro-OmpA derivatives with a His6 tag in their N-terminal "translocation initiation domains" are arrested by Ni2+ at an early post-targeting stage of translocation

We examined in vitro translocation of pro-OmpA derivatives with a His6 tag at various positions in their mature proteins and with a c-Myc tag at their C termini across inverted membrane vesicles of Escherichia coli. Those with a His6 tag in the N-terminal region of the mature domain, which corresponds to the "translocation initiation domain" proposed previously (Andersson, H., and von Heijne, G. (1991) Proc. Natl. Acad. Sci. U. S. A. 88, 9751-9754), could not be translocated in the presence of 100 micron Ni2+, while OmpA derivatives with a His6 tag in the middle of or at the C terminus did not show such Ni2+ sensitivity. The inhibitory action of Ni2+ on pro-3His-OmpA' (with a His6 tag after the third amino acid of the mature OmpA-c-Myc region) translocation was exerted only during early events, after which it became ineffective. The inhibition point of Ni2+ was suggested to lie between membrane targeting and exposure of the signal cleavage site to the periplasm since the unprocessed and membrane-bound form of pro-3His-OmpA' was accumulated by the addition of Ni2+. The Ni(2+)-"trapped" precursor was released from its translocation block by 30 mM histidine, which should compete with the His6 tag on the precursor protein for formation of a Ni2+ chelating complex. We propose that Ni2+ confers a reversible positive charge effect on the His6-tagged initiation domain of the pro-OmpA derivatives and inhibits an early event(s) of protein translocation, such as presentation of the precursor to the membranous part of the translocase. This system will be useful in dissecting early events of the protein translocation pathway.