DIFFERENT PATHWAYS FOR PROTEIN-DEGRADATION BY THE FTSH HFLKC MEMBRANE-EMBEDDED PROTEASE COMPLEX - AN IMPLICATION FROM THE INTERFERENCE BY AMUTANT FORM OF A NEW SUBSTRATE PROTEIN, YCCA/

Escherichia coli FtsH (HflB) is a membrane-bound and ATP-dependent zin c-metalloproteinase, which forms a complex with a pair of periplasmica lly exposed membrane proteins, HflK and HflC. It is the protease that degrades uncomplexed forms of the SecY subunit of protein translocase. Here,we characterized a new class of SecY-stabilizing mutation on the E. coli chromosome. The mutation (yccA11) is an internal deletion wit hin a gene (yccA) known as an open reading frame for a hydrophobic pro tein with putative seven transmembrane segments. The YccA protein was found to be degraded in an FtsH-dependent manner in vivo and in vitro, whereas the YccA11 mutant protein, lacking eight amino acid residues within the amino-terminal cytoplasmic domain, was refractory to the de gradation. The yccA11 mutation exhibited partial dominance when overex pressed. Cross-linking co-immunoprecipitation, and histidine tagging e xperiments showed that YccA11 as well as YccA can associate with both the FtsH and the HflKC proteins. Thus, the mutant YccA protein appeare d to compete with SecY for recognition by the FtsH proteolytic system and the residues deleted by the yccA mutation are required for the ini tiation of proteolysis by FtsH. Interestingly, the inhibitory action o f YccA11 was mediated by HflKC, since the deletion of hflK-hflC suppre ssed the yccA11 phenotype. The yccA11 mutation stabilized subunit a of the proton ATPase F-0 segment as well, but not the CII protein of bac teriophage lambda or the sigma(32) protein. From these results we sugg est that there are at least two pathways for FtsH-deyendent protein de gradation, only one of which (probably for membrane proteins) is subje ct to the HflKC-dependent interference by the YccA11 mutant substrate. (C) 1998 Academic Press Limited.