Syd, a SecY-interacting protein, excludes SecA from the SecYE complex with an altered SecY24 subunit

The Dynamic SecYEG Translocon

The spatial and temporal coordination of protein transport is an essential cornerstone of the bacterial adaptation to different environmental conditions. By adjusting the protein composition of extra-cytosolic compartments, like the inner and outer membranes or the periplasmic space, protein transport mechanisms help shaping protein homeostasis in response to various metabolic cues. The universally conserved SecYEG translocon acts at the center of bacterial protein transport and mediates the translocation of newly synthesized proteins into and across the cytoplasmic membrane. The ability of the SecYEG translocon to transport an enormous variety of different substrates is in part determined by its ability to interact with multiple targeting factors, chaperones and accessory proteins. These interactions are crucial for the assisted passage of newly synthesized proteins from the cytosol into the different bacterial compartments. In this review, we summarize the current knowledge about SecYEG-mediated protein transport, primarily in the model organism Escherichia coli, and describe the dynamic interaction of the SecYEG translocon with its multiple partner proteins. We furthermore highlight how protein transport is regulated and explore recent developments in using the SecYEG translocon as an antimicrobial target.

Identification and characterization of a translation arrest motif in VemP by systematic mutational analysis

VemP ( Vibrio protein export monitoring polypeptide) is a secretory protein comprising 159 amino acid residues, which functions as a secretion monitor in Vibrio and regulates expression of the downstream V.secDF2 genes. When VemP export is compromised, its translation specifically undergoes elongation arrest at the position where the Gln¹⁵⁶ codon of vemP encounters the P-site in the translating ribosome, resulting in up-regulation of V.SecDF2 production. Although our previous study suggests that many residues in a highly conserved C-terminal 20-residue region of VemP contribute to its elongation arrest, the exact role of each residue remains unclear. Here, we constructed a reporter system to easily and exactly monitor the in vivo arrest efficiency of VemP. Using this reporter system, we systematically performed a mutational analysis of the 20 residues (His¹³⁸-Phe¹⁵⁷) to identify and characterize the arrest motif. Our results show that 15 residues in the conserved region participate in elongation arrest and that multiple interactions between important residues in VemP and in the interior of the exit tunnel contribute to the elongation arrest of VemP. The arrangement of these important residues induced by specific secondary structures in the ribosomal tunnel is critical for the arrest. Pro scanning analysis of the preceding segment (Met¹²⁰-Phe¹³⁷) revealed a minor role of this region in the arrest. Considering these results, we conclude that the arrest motif in VemP is mainly composed of the highly conserved multiple residues in the C-terminal region.

Conformational variation of the translocon enhancing chaperone SecDF

The Sec translocon facilitates transportation of newly synthesized polypeptides from the cytoplasm to the lumen/periplasm across the phospholipid membrane. Although the polypeptide-conducting machinery is formed by the SecYEG-SecA complex in bacteria, its transportation efficiency is markedly enhanced by SecDF. A previous study suggested that SecDF assumes at least two conformations differing by a 120° rotation in the spatial orientation of the P1 head subdomain to the rigid base, and that the conformational dynamics plays a critical role in polypeptide translocation. Here we addressed this hypothesis by analyzing the 3D structure of SecDF using electron tomography and single particle reconstruction. Reconstruction of wt SecDF showed two major conformations; one resembles the crystal structure of full-length SecDF (F-form structure), while the other is similar to the hypothetical structural variant based on the crystal structure of the isolated P1 domain (I-form structure). The transmembrane domain of the I-form structure has a scissor like cleft open to the periplasmic side. We also report the structure of a double cysteine mutant designed to constrain SecDF to the I-form. This reconstruction has a protrusion at the periplasmic end that nicely fits the orientation of P1 in the I-from. These results provide firm evidence for the occurrence of the I-form in solution and support the proposed F- to I-transition of wt SecDF during polypeptide translocation.

Structure and function of a membrane component SecDF that enhances protein export

Protein translocation across the bacterial membrane, mediated by the secretory translocon SecYEG and the SecA ATPase, is enhanced by proton motive force and membrane-integrated SecDF, which associates with SecYEG. The role of SecDF has remained unclear, although it is proposed to function in later stages of translocation as well as in membrane protein biogenesis. Here, we determined the crystal structure of Thermus thermophilus SecDF at 3.3 Å resolution, revealing a pseudo-symmetrical, 12-helix transmembrane domain belonging to the RND superfamily and two major periplasmic domains, P1 and P4. Higher-resolution analysis of the periplasmic domains suggested that P1, which binds an unfolded protein, undergoes functionally important conformational changes. In vitro analyses identified an ATP-independent step of protein translocation that requires both SecDF and proton motive force. Electrophysiological analyses revealed that SecDF conducts protons in a manner dependent on pH and the presence of an unfolded protein, with conserved Asp and Arg residues at the transmembrane interface between SecD and SecF playing essential roles in the movements of protons and preproteins. Therefore, we propose that SecDF functions as a membrane-integrated chaperone, powered by proton motive force, to achieve ATP-independent protein translocation.

A novel immunity system for bacterial nucleic acid degrading toxins and its recruitment in various eukaryotic and DNA viral systems

The use of nucleases as toxins for defense, offense or addiction of selfish elements is widely encountered across all life forms. Using sensitive sequence profile analysis methods, we characterize a novel superfamily (the SUKH superfamily) that unites a diverse group of proteins including Smi1/Knr4, PGs2, FBXO3, SKIP16, Syd, herpesviral US22, IRS1 and TRS1, and their bacterial homologs. Using contextual analysis we present evidence that the bacterial members of this superfamily are potential immunity proteins for a variety of toxin systems that also include the recently characterized contact-dependent inhibition (CDI) systems of proteobacteria. By analyzing the toxin proteins encoded in the neighborhood of the SUKH superfamily we predict that they possess domains belonging to diverse nuclease and nucleic acid deaminase families. These include at least eight distinct types of DNases belonging to HNH/EndoVII- and restriction endonuclease-fold, and RNases of the EndoU-like and colicin E3-like cytotoxic RNases-folds. The N-terminal domains of these toxins indicate that they are extruded by several distinct secretory mechanisms such as the two-partner system (shared with the CDI systems) in proteobacteria, ESAT-6/WXG-like ATP-dependent secretory systems in Gram-positive bacteria and the conventional Sec-dependent system in several bacterial lineages. The hedgehog-intein domain might also release a subset of toxic nuclease domains through auto-proteolytic action. Unlike classical colicin-like nuclease toxins, the overwhelming majority of toxin systems with the SUKH superfamily is chromosomally encoded and appears to have diversified through a recombination process combining different C-terminal nuclease domains to N-terminal secretion-related domains. Across the bacterial superkingdom these systems might participate in discriminating `self’ or kin from `non-self’ or non-kin strains. Using structural analysis we demonstrate that the SUKH domain possesses a versatile scaffold that can be used to bind a wide range of protein partners. In eukaryotes it appears to have been recruited as an adaptor to regulate modification of proteins by ubiquitination or polyglutamylation. Similarly, another widespread immunity protein from these toxin systems, namely the suppressor of fused (SuFu) superfamily has been recruited for comparable roles in eukaryotes. In animal DNA viruses, such as herpesviruses, poxviruses, iridoviruses and adenoviruses, the ability of the SUKH domain to bind diverse targets has been deployed to counter diverse anti-viral responses by interacting with specific host proteins.

Structure, binding, and activity of Syd, a SecY-interacting protein

The Syd protein has been implicated in the Sec-dependent transport of polypeptides across the bacterial inner membrane. Using Nanodiscs, we here provide direct evidence that Syd binds the SecY complex, and we demonstrate that interaction involves the two electropositive and cytosolic loops of the SecY subunit. We solve the crystal structure of Syd and together with cysteine cross-link analysis, we show that a conserved concave and electronegative groove constitutes the SecY-binding site. At the membrane, Syd decreases the activity of the translocon containing loosely associated SecY-SecE subunits, whereas in detergent solution Syd disrupts the SecYEG heterotrimeric associations. These results support the role of Syd in proofreading the SecY complex biogenesis and point to the electrostatic nature of the Sec channel interaction with its cytosolic partners.

The Long α-Helix of SecA Is Important for the ATPase Coupling of Translocation

SecA contains two ATPase folds (NBF1 and NBF2) and other interaction/regulatory domains, all of which are connected by a long helical scaffold domain (HSD) running along the molecule. Here we identified a functionally important and spatially adjacent pair of SecA residues, Arg-642 on HSD and Glu-400 on NBF1. A charge-reversing substitution at either position as well as disulfide tethering of these positions inactivated the translocation activity. Interestingly, however, the translocation-inactive SecA variants fully retained the ability to up-regulate the ATPase in response to a preprotein and the SecYEG translocon. The translocation defect was suppressible by second site alterations at the hinge-forming boundary of NBF2 and HSD. Based on these results, we propose that the motor function of SecA is realized by ligand-activated ATPase engine and its HSD-mediated conversion into the mechanical work of preprotein translocation.

Purification, crystallization and preliminary X-ray diffraction of SecDF, a translocon-associated membrane protein, from Thermus thermophilus

Thermus thermophilus has a multi-path membrane protein, TSecDF, as a single-chain homologue of Escherichia coli SecD and SecF, which form a translocon-associated complex required for efficient preprotein translocation and membrane-protein integration. Here, the cloning, expression in E. coli, purification and crystallization of TSecDF are reported. Overproduced TSecDF was solubilized with dodecylmaltoside, chromatographically purified and crystallized by vapour diffusion in the presence of polyethylene glycol. The crystals yielded a maximum resolution of 4.2 angstroms upon X-ray irradiation, revealing that they belonged to space group P4(3)2(1)2. Attempts were made to improve the diffraction quality of the crystals by combinations of micro-stirring, laser-light irradiation and dehydration, which led to the eventual collection of complete data sets at 3.74 angstroms resolution and preliminary success in the single-wavelength anomalous dispersion analysis. These results provide information that is essential for the determination of the three-dimensional structure of this important membrane component of the protein-translocation machinery.

Structure and function of SecA, the preprotein translocase nanomotor

Most secretory proteins that are destined for the periplasm or the outer membrane are exported through the bacterial plasma membrane by the Sec translocase. Translocase is a complex nanomachine that moves processively along its aminoacyl polymeric substrates effectively pumping them to the periplasmic space. The salient features of this process are: (a) a membrane-embedded "clamp" formed by the trimeric SecYEG protein, (b) a "motor" provided by the dimeric SecA ATPase, (c) regulatory subunits that optimize catalysis and (d) both chemical and electrochemical metabolic energy. Significant recent strides have allowed structural, biochemical and biophysical dissection of the export reaction. A model incorporating stepwise strokes of the translocase nanomachine at work is discussed.

Nearest neighbor analysis of the SecYEG complex. 1. Identification of a SecY-SecG interface

Integral membrane components SecY, SecE, and SecG of protein translocase form a complex in the Escherichia coli plasma membrane. To characterize subunit interactions of the SecYEG complex, a series of SecY variants having a single cysteine in its cytoplasmic (C1-C6) or periplasmic (P1-P5) domain were subjected to site-specific cross-linking experiments using bifunctional agents with thiol-amine reactivity. Experiments using inverted membrane vesicles revealed specific cross-linkings between a cysteine residue placed in the C2 or C3 domain of SecY and the cytosolic lysine (Lys26) near the first transmembrane segment of SecG. These SecY Cys residues also formed a disulfide bond with an engineered cytosolic cysteine at position 28 of SecG. Thus, the C2-C3 region of SecY is in the proximity of the N-terminal half of the SecG cytoplasmic loop. Experiments using spheroplasts revealed the physical proximity of P2 (SecY) and the C-terminal periplasmic region of SecG. In addition, mutations in secG were isolated as suppressors against a cold-sensitive mutation (secY104) affecting the TM4-C3 boundary of SecY. These results collectively suggest that a C2-TM3-P2-TM4-C3 region of SecY serves as an interface with SecG.

Nearest neighbor analysis of the SecYEG complex. 2. Identification of a SecY-SecE cytosolic interface

Although the importance of interactions involving both the cytosolic and transmembrane regions of SecY and SecE has been documented, no information has been available for the physical contact sites of these translocase subunits in their cytosolic domains. We now carried out site-specific cross-linking experiments to identify SecY and SecE regions that are physically close. Cysteines introduced into SecY residue 244 in the fourth cytosolic domain (C4) as well as into residues 354-356 and 362 in the C5 domain could be cross-linked with natural or engineered residues at positions 79 and 81 in the central part of the cytosolic loop of SecE. These cross-linkages were abolished by the Gly240 mutation in the SecY C4 region as well as by prlG alterations in SecE transmembrane segment 3, known to compromise SecY-SecE interaction. We suggest that the cytosolic and intramembrane interactions bring these two subunits together, forming a functionally crucial SecYE interface involving the SecY C5 region and the conserved cytosolic segment of SecE.

Importance of transmembrane segments in Escherichia coli SecY

To assess the functional importance of the transmembrane regions of SecY, we constructed a series of SecY variants, in which the six central residues of each transmembrane segment were replaced by amino acid residues from either transmembrane segment 3 or 4 of LacY. The SecY function, as assessed by the ability to complement cold-sensitive secYmutants with respect to their growth and translocase defects, was eliminated by the alterations in transmembrane segments 2, 3, 4, 7, 9 and 10. Among them, those in segments 3 and 4 had especially severe effects. In contrast, transmembrane segments 1, 5, 6, and 8 were more tolerant to the sequence alterations. The purified protein with an altered transmembrane segment 6 retained, in large measure, the ability to support SecA-dependent preprotein translocation in vitro. These results will help us to further understand how the SecYEG protein translocation channel functions.

Fluorescence resonance energy transfer analysis of protein translocase. SecYE from Thermus thermophilus HB8 forms a constitutive oligomer in membranes

SecY and SecE are the two principal translocase subunits that create a channel-like pathway for the transit of preprotein across the bacterial cytoplasmic membrane. Here we report the cloning, expression, and purification of the SecYE complex (TSecYE) from a thermophilic bacterium, Thermus thermophilus HB8. Purified TSecYE can be reconstituted into proteoliposomes that function in T. thermophilus SecA (TSecA) dependent preprotein translocation. After the mixing of TSecYE derivatives labeled with either a donor or an acceptor fluorophore during reconstitution, fluorescence resonance energy transfer experiments demonstrated that 2 or more units of TSecYE in the lipid bilayer associate to form a largely non-exchangeable oligomeric structure.

Interfering mutations provide in vivo evidence that Escherichia coli SecE functions in multimeric states

SecY, SecE and SecG form a heterotrimer, which functions as a protein translocation channel in Escherichia coli. The cytosolic loop of SecE contains a segment that is conserved among different organisms. Here we show that mutational alterations in this segment not only inactivate the SecE function but confer dominant interfering properties on the altered SecE molecule. Such effects were especially evident in mutant cells in which the requirement for SecE function was increased. Overproduction of SecE, but not of SecY, alleviated the dominant negative effects. These results suggest that the inactive SecE molecule sequesters wild-type SecE. It was also found that an amino acid substitution, D112P, in the C-terminal periplasmic region intragenically suppressed the dominant interference. These results are consistent with a notion that there is significant SecE-SecE interaction in vivo, in which the C-terminal region has an important role. The data hence suggest that dimeric SecE participates in the formation of the functional translocation channel.

Biochemical characterization of a mutationally altered protein translocase: proton motive force stimulation of the initiation phase of translocation

Protein translocation across the Escherichia coli plasma membrane is facilitated by concerted actions of the SecYEG integral membrane complex and the SecA ATPase. A secY mutation (secY39) affects Arg357, an evolutionarily conserved and functionally important residue, and impairs the translocation function in vivo and in vitro. In this study, we used the "superactive" mutant forms of SecA, which suppress the SecY39 deficiency, to characterize the mutationally altered SecY39EG translocase. It was found that SecY39-mediated preprotein translocation exhibited absolute dependence on the proton motive force. The proton motive force-dependent step proved to lie before signal peptide cleavage. We suggest that the proton motive force assists in the initiation phase of protein translocation.

Superactive SecY variants that fulfill the essential translocation function with a reduced cellular quantity

The fifth and the sixth cytoplasmic regions (C5 and C6) of SecY are important for the SecA-driven preprotein translocation reaction. A cold-sensitive mutation, secY205 (Tyr-429 --> Asp), in C6 impairs the ATP- and precursor-dependent SecA insertion into the membrane. We now identified second site mutations that suppressed the defect. Cis-placement of these mutations proved to suppress mutations at another essential residue (Arg-357) of SecY as well. Thus, they tolerate the otherwise defective SecY alterations in the same molecule. Two alterations (Ile-195 to Ser in TM5 region and Ile-408 to Leu in TM10 region) were found to make the translocation channel more active, because it enabled cells to survive with reduced content of the SecYE complex. These mutations only very weakly suppressed a signal sequence defect of the lambda receptor protein. The mutant SecYEG translocase exhibited higher than normal activity in vitro, being accompanied by striking independence of the proton motive force as well as by stabilization of a bound and active SecA species against urea treatment. These results have been interpreted in terms of balance shifts between channel closing and channel opening alterations in the SecYEG translocase.

Membrane protein degradation by FtsH can be initiated from either end

FtsH, a membrane-bound metalloprotease, with cytoplasmic metalloprotease and AAA ATPase domains, degrades both soluble and integral membrane proteins in Escherichia coli. In this paper we investigated how membrane-embedded substrates are recognized by this enzyme. We showed previously that FtsH can initiate processive proteolysis at an N-terminal cytosolic tail of a membrane protein, by recognizing its length (more than 20 amino acid residues) but not exact sequence. Subsequent proteolysis should involve dislocation of the substrates into the cytosol. We now show that this enzyme can also initiate proteolysis at a C-terminal cytosolic tail and that the initiation efficiency depends on the length of the tail. This mode of degradation also appeared to be processive, which can be aborted by a tightly folded periplasmic domain. These results indicate that FtsH can exhibit processivity against membrane-embedded substrates in either the N-to-C or C-to-N direction. Our results also suggest that some membrane proteins receive bidirectional degradation simultaneously. These results raise intriguing questions about the molecular directionality of the dislocation and proteolysis catalyzed by FtsH.

Roles of the C-terminal end of SecY in protein translocation and viability of Escherichia coli

SecY, a central component of the membrane-embedded sector of protein translocase, contains six cytosolic domains. Here, we examined the importance of the C-terminal cytosolic region of SecY by systematically shortening the C-terminal end and examining the functional consequences of these mutations in vivo and in vitro. It was indicated that the C-terminal five residues are dispensable without any appreciable functional defects in SecY. Mutants missing the C-terminal six to seven residues were partially compromised, especially at low temperature or in the absence of SecG. In vitro analyses indicated that the initial phase of the translocation reaction, in which the signal sequence region of the preprotein is inserted into the membrane, was affected by the lack of the C-terminal residues. SecA binding was normal, but SecA insertion in response to ATP and a preprotein was impaired. It is suggested that the C-terminal SecY residues are required for SecA-dependent translocation initiation.

The Sec protein-translocation pathway

The Sec machinery (or translocase) provides a major pathway of protein translocation from the cytosol across the cytoplasmic membrane in bacteria. The SecA ATPase interacts dynamically with the SecYEG integral membrane components to drive the transmembrane movement of newly synthesized preproteins. This pathway is also used for integration of some membrane proteins and the Sec translocase interacts with other cellular components to achieve its cellular roles. The detailed protein interactions involved in these processes are being actively studied and a structural understanding of the protein-conducting channel has started to emerge.

Interim report on genomics of Escherichia coli

We present a summary of recent progress in understanding Escherichia coli K-12 gene and protein functions. New information has come both from classical biological experimentation and from using the analytical tools of functional genomics. The content of the E. coli genome can clearly be seen to contain elements acquired by horizontal transfer. Nevertheless, there is probably a large, stable core of >3500 genes that are shared among all E. coli strains. The gene-enzyme relationship is examined, and, in many cases, it exhibits complexity beyond a simple one-to-one relationship. Also, the E. coli genome can now be seen to contain many multiple enzymes that carry out the same or closely similar reactions. Some are similar in sequence and may share common ancestry; some are not. We discuss the concept of a minimal genome as being variable among organisms and obligatorily linked to their life styles and defined environmental conditions. We also address classification of functions of gene products and avenues of insight into the history of protein evolution.

Protein traffic in bacteria: multiple routes from the ribosome to and across the membrane

Bacteria use several routes to target their exported proteins to the plasma membrane. The majority are exported through pores formed by SecY and SecE. Two different molecular machineries are used to target proteins to the SecYE translocon. Translocated proteins, synthesized as precursors with cleavable signal sequences, require cytoplasmic chaperones, such as SecB, to remain competent for posttranslational transport. In concert with SecB, SecA targets the precursors to SecY and energizes their translocation by its ATPase activity. The latter function involves a partial insertion of SecA itself into the SecYE translocon, a process that is strongly assisted by a couple of membrane proteins, SecG, SecD, SecF, YajC, and the proton gradient across the membrane. Integral membrane proteins, however, are specifically recognized by a direct interaction between their noncleaved signal anchor sequences and the bacterial signal recognition particle (SRP) consisting of Ffh and 4.5S RNA. Recognition occurs during synthesis at the ribosome and leads to a cotranslational targeting to SecYE that is mediated by FtsY and the hydrolysis of GTP. No other Sec protein is required for integration unless the membrane protein also contains long translocated domains that engage the SecA machinery. Discrimination between SecA/SecB- and SRP-dependent targeting involves the specificity of SRP for hydrophobic signal anchor sequences and the exclusion of SRP from nascent chains of translocated proteins by trigger factor, a ribosome-associated chaperone. The SecYE pore accepts only unfolded proteins. In contrast, a class of redox factor-containing proteins leaves the cell only as completely folded proteins. They are distinguished by a twin arginine motif of their signal sequences that by an unknown mechanism targets them to specific pores. A few membrane proteins insert spontaneously into the bacterial plasma membrane without the need for targeting factors and SecYE. Insertion depends only on hydrophobic interactions between their transmembrane segments and the lipid bilayer and on the transmembrane potential. Finally, outer membrane proteins of Gram-negative bacteria after having crossed the plasma membrane are released into the periplasm, where they undergo distinct folding events until they insert as trimers into the outer membrane. These folding processes require distinct molecular chaperones of the periplasm, such as Skp, SurA, and PpiD.

Secretion monitor, SecM, undergoes self-translation arrest in the cytosol

The product of the Escherichia coli secM gene (secretion monitor, formerly gene X), upstream of secA, is involved in secretion-responsive control of SecA translation. In wild-type cells, SecM is rapidly degraded by the periplasmic tail-specific protease. It is also subject to a transient translation pause at a position close to the C terminus. The elongation arrest was strikingly prolonged when translocation of SecM was impaired. SRP was not required for this arrest. Instead, the nascent SecM product itself may participate, as the arrest was diminished when it incorporated a proline analog, azetidine. We propose that cytosolically localized nascent SecM undergoes self-translation arrest, thereby enhancing translation of secA through an altered secondary structure of the secM-secA messenger RNA.

Genetic dissection of SecA: suppressor mutations against the secY205 translocase defect

BACKGROUND

The driving force for protein translocation across the bacterial plasma membrane is provided by SecA ATPase, which undergoes striking conformational changes characterized by the membrane insertion and deinsertion cycle. This action of SecA requires the membrane-embedded SecYEG complex. Previously, we have identified a cold-sensitive secY mutation (secY205), affecting the most carboxy-terminal cytosolic domain, that did not allow an ATP-dependent insertion of a SecA-preprotein complex. Thus, this mutant provides an excellent system for genetic analysis of the SecY-SecA interaction.

RESULTS

We carried out a systematic isolation of secA mutations that suppressed secY205 cold-sensitivity. A total of 40 independent suppressor mutations were classified into: (i) allele-specific suppressors, acting only against secY205, and (ii) 'super active' suppressors, acting against almost any sec defects. The former class of mutations, presumably with specific effects on the SecY-SecA interaction, clustered in two regions close to the Walker motif A sequences of the two ATP-binding domains. The latter mutations, enhancing general SecA activities, were mostly in or around the minor ATP-binding domain.

CONCLUSIONS

The Walker motif A regions of SecA are important for the SecA-SecY interaction that leads to the SecA conformational changes required for insertion into the SecYEG channel. The minor ATP-binding domain is important for the down-regulation of SecA activities.

Length recognition at the N-terminal tail for the initiation of FtsH-mediated proteolysis

FtsH-mediated proteolysis against membrane proteins is processive, and presumably involves dislocation of the substrate into the cytosol where the enzymatic domains of FtsH reside. To study how such a mode of proteolysis is initiated, we manipulated N-terminal cytosolic tails of three membrane proteins. YccA, a natural substrate of FtsH was found to require the N-terminal tail of 20 amino acid residues or longer to be degraded by FtsH in vivo. Three unrelated sequences of this segment conferred the FtsH sensitivity to YccA. An artificially constructed TM9-PhoA protein, derived from SecY, as well as the SecE protein, were sensitized to FtsH by addition of extra amino acid sequences to their N-terminal cytosolic tails. Thus, FtsH recognizes a cytosolic region of sufficient length (approximately 20 amino acids) to initiate the processive proteolysis against membrane proteins. Such a region is typically at the N-terminus and can be diverse in amino acid sequences.

A mutation in secY that causes enhanced SecA insertion and impaired late functions in protein translocation

A cold-sensitive secY mutant (secY125) with an amino acid substitution in the first periplasmic domain causes in vivo retardation of protein export. Inverted membrane vesicles prepared from this mutant were as active as the wild-type membrane vesicles in translocation of a minute amount of radioactive preprotein. The mutant membrane also allowed enhanced insertion of SecA, and this SecA insertion was dependent on the SecD and SecF functions. These and other observations suggested that the early events in translocation, such as SecA-dependent insertion of the signal sequence region, is actually enhanced by the SecY125 alteration. In contrast, since the mutant membrane vesicles had decreased capacity to translocate chemical quantity of pro-OmpA and since they were readily inactivated by pretreatment of the vesicles under the conditions in which a pro-OmpA translocation intermediate once accumulated, the late translocation functions appear to be impaired. We conclude that this periplasmic secY mutation causes unbalanced early and late functions in translocation, compromising the translocase's ability to catalyze multiple rounds of reactions.