Simultaneous formation of right- and left-handed anti-parallel coiled-coil interfaces by a coil2 fragment of human lamin A

The elementary building block of all intermediate filaments (IFs) is a dimer featuring a central α-helical rod domain flanked by the N- and C-terminal end domains. In nuclear IF proteins (lamins), the rod domain consists of two coiled-coil segments, coil1 and coil2, that are connected by a short non-helical linker. Coil1 and the C-terminal part of coil2 contain the two highly conserved IF consensus motifs involved in the longitudinal assembly of dimers. The previously solved crystal structure of a lamin A fragment (residues 305-387) corresponding to the second half of coil2 has yielded a parallel left-handed coiled coil. Here, we present the crystal structure and solution properties of another human lamin A fragment (residues 328-398), which is largely overlapping with fragment 305-387 but harbors a short segment of the tail domain. Unexpectedly, no parallel coiled coil forms within the crystal. Instead, the α-helices are arranged such that two anti-parallel coiled-coil interfaces are formed. The most significant interface has a right-handed geometry, which is accounted for by a characteristic 15-residue repeat pattern that overlays with the canonical heptad repeat pattern. The second interface is a left-handed anti-parallel coiled coil based on the predicted heptad repeat pattern. In solution, the fragment reveals only a weak dimerization propensity. We speculate that the C-terminus of coil2 might unzip, thereby allowing for a right-handed coiled-coil interface to form between two laterally aligned dimers. Such an interface might co-exist with a heterotetrameric left-handed coiled-coil assembly, which is expected to be responsible for the longitudinal A_CN contact.     

Structure of the type III secretion recognition protein YscU from Yersinia enterocolitica

The inner-membrane protein YscU has an important role during the assembly of the Yersinia enterocolitica type III secretion injectisome. Its cytoplasmic domain (YscU^C) recognizes translocators as individual substrates in the export hierarchy. Activation of YscU entails autocleavage at a conserved NPTH motif. Modification of this motif markedly changes the properties of YscU, including translocator export cessation and production of longer injectisome needles. We determined the crystal structures of the uncleaved variants N263A and N263D of YscU^C at 2.05 Å and 1.55 Å resolution, respectively. The globular domain is found to consist of a central, mixed β-sheet surrounded by α-helices. The NPTH motif forms a type II β-turn connecting two β-strands. NMR analysis of cleaved and uncleaved YscU^C indicates that the global structure of the protein is retained in cleaved YscU^C. The structure of YscU^C variant N263D reveals that wild type YscU^C is poised for cleavage due to an optimal reaction geometry for nucleophilic attack of the scissile bond by the side chain of Asn263. In vivo analysis of N263Q and H266A/R314A YscU variants showed a phenotype that combines the absence of translocator secretion with normal needle-length control. Comparing the structure of YscU to those of related proteins reveals that the linker domain between the N-terminal transmembrane domain and the autocleavage domain can switch from an extended to a largely α-helical conformation, allowing for optimal positioning of the autocleavage domain during injectisome assembly.     

Crystal structure of a membrane stomatin-specific protease in complex with a substrate peptide

Membrane-bound proteases are involved in various regulatory functions. A previous report indicated that the N-terminal region of PH1510p (1510-N) from the hyperthermophilic archaeon Pyrococcus horikoshii is a serine protease with a catalytic Ser-Lys dyad (Ser97 and Lys138) and specifically cleaves the C-terminal hydrophobic region of the p-stomatin PH1511p. In humans, an absence of stomatin is associated with a form of hemolytic anemia known as hereditary stomatocytosis. Here, the crystal structure of 1510-N K138A in complex with a peptide substrate was determined at 2.25 ? resolution. In the structure, a 1510-N dimer binds to one peptide. The six central residues (VIVLML) of the peptide are hydrophobic and in a pseudopalindromic structure and therefore favorably fit into the hydrophobic active tunnel of the 1510-N dimer, although 1510-N degrades the substrate at only one point. A comparison with unliganded 1510-N K138A revealed that the binding of the substrate causes a large rotational and translational displacement between protomers and produces a tunnel suitable for binding the peptide. When the peptide binds, the flexible L2 loop of one protomer forms β-strands, whereas that of the other protomer remains in a loop form, indicating that one protomer binds to the peptide more tightly than the other protomer. The Ala138 residues of the two protomers are located very close together (the distance between the two Cβ atoms is 3.6 ?). Thus, in wild-type 1510-N, the close positioning of the catalytic Ser97 and Lys138 residues may be induced by electrostatic repulsion of the two Lys138 side chains of the protomers.     

Structure of the periplasmic adaptor protein from a major facilitator superfamily (MFS) multidrug efflux pump

Periplasmic adaptor proteins are key components of bacterial tripartite efflux pumps. The 2.85 Å resolution structure of an MFS (major facilitator superfamily) pump adaptor, Aquifex aeolicus EmrA, shows linearly arranged α-helical coiled-coil, lipoyl, and β-barrel domains, but lacks the fourth membrane-proximal domain shown in other pumps to interact with the inner membrane transporter. The adaptor α-hairpin, which binds outer membrane TolC, is exceptionally long at 127 Å, and the β-barrel contains a conserved disordered loop. The structure extends the view of adaptors as flexible, modular components that mediate diverse pump assembly, and suggests that in MFS tripartite pumps a hexamer of adaptors could provide a periplasmic seal.     

An Evolutionarily Conserved Family of Virion Tail Needles Related to Bacteriophage P22 gp26: Correlation between Structural Stability and Length of the α-Helical Trimeric Coiled Coil

Bacteriophages of the Podoviridae family use short noncontractile tails to inject their genetic material into Gram-negative bacteria. In phage P22, the tail contains a thin needle, encoded by the phage gene 26, which is essential both for stabilization and for ejection of the packaged viral genome. Bioinformatic analysis of the N-terminal domain of gp26 (residues 1-60) led us to identify a family of genes encoding putative homologues of the tail needle gp26. To validate this idea experimentally and to explore their diversity, we cloned the gp26-like gene from phages HK620, Sf6 and HS1, and characterized these gene products in solution. All gp26-like factors contain an elongated α-helical coiled-coil core consisting of repeating, adjacent trimerization heptads and form trimeric fibers with length ranging between about 240 to 300 Å. gp26 tail needles display a high level of structural stability in solution, with T_m (temperature of melting) between 85 and 95 °C. To determine how the structural stability of these phage fibers correlates with the length of the α-helical core, we investigated the effect of insertions and deletions in the helical core. In the P22 tail needle, we identified an 85-residue-long helical domain, termed MiCRU (minimal coiled-coil repeat unit), that can be inserted in-frame inside the gp26 helical core, preserving the straight morphology of the fiber. Likewise, we were able to remove three quarters of the helical core of the HS1 tail needle, minimally decreasing the stability of the fiber. We conclude that in the gp26 family of tail needles, structural stability increases nonlinearly with the length of the α-helical core. Thus, the overall stability of these bacteriophage fibers is not solely dependent on the number of trimerization repeats in the α-helical core.     

Possible formation of a triple-stranded coiled-coil region in tropomyosin-troponin T binding complex

Immunoelectron microscopy has shown that the spatial arrangement of troponin T on tropomyosin can be represented as a structure of approximately 90 Å in length, as shown in Figure 1. The region of residues 90 to 148 of troponin T, which has been confirmed as a main part of the fragment which specifically binds to tropomyosin, was predicted to be a long stretch of α-helix by the method of secondary structure prediction. Furthermore, the mechanism of the specific binding was explored on the basis of the coiled-coil structure of tropomyosin by a simple scoring method. One of the most feasible structures of the specific binding complex was a triple-stranded coiled-coil made between a tropomyosin coiled-coil and the α-helical region of the specific binding fragment of troponin T. It is illustrated as a stereo view in Figure 2.     

Crystal structure of the cytoplasmic N-terminal domain of subunit I, a homolog of subunit a, of V-ATPase

Subunit “a” is associated with the membrane-bound (V_O) complex of eukaryotic vacuolar H⁺-ATPase acidification machinery. It has also been shown recently to be involved in diverse membrane fusion/secretory functions independent of acidification. Here, we report the crystal structure of the N-terminal cytosolic domain from the Meiothermus ruber subunit “I” homolog of subunit a. The structure is composed of a curved long central α-helix bundle capped on both ends by two lobes with similar α/β architecture. Based on the structure, a reasonable model of its eukaryotic subunit a counterpart was obtained. The crystal structure and model fit well into reconstructions from electron microscopy of prokaryotic and eukaryotic vacuolar H⁺-ATPases, respectively, clarifying their orientations and interactions and revealing features that could enable subunit a to play a role in membrane fusion/secretion.     

High-resolution structure of ybfF from Escherichia coli K12: a unique substrate-binding crevice generated by domain arrangement

Esterases are one of the most common enzymes and are involved in diverse cellular functions. ybfF protein from Escherichia coli (Ec_ybfF) belongs to the esterase family for the large substrates, palmitoyl coenzyme A and malonyl coenzyme A, which are important cellular intermediates for energy conversion and biomolecular synthesis. To obtain molecular information on ybfF esterase, which is found in a wide range of microorganisms, we elucidated the crystal structures of Ec_ybfF in complexes with small molecules at resolutions of 1.1 and 1.68 Å, respectively. The structure of Ec_ybfF is composed of a globular α/β hydrolase domain with a three-helical bundle cap, which is linked by a kinked helix to the α/β hydrolase domain. It contains a catalytic tetrad of Ser-His-Asp-Ser with the first Ser acting as a nucleophile. The unique spatial arrangement and orientation of the helical cap with respect to the α/β hydrolase domain form a substrate-binding crevice for large substrates. The helical cap is also directly involved in catalysis by providing a substrate anchor, viz., the conserved residues of Arg123 and Tyr208. The high-resolution structure of Ec_ybfF shows that the inserted helical bundle structure and its spatial orientation with respect to the α/β hydrolase domain are critical for creating a large inner space and constituting a specific active site, thereby providing the broad substrate spectrum toward large biomolecules.     

A cell-penetrating helical peptide as a potential HIV-1 inhibitor

The capsid domain of the human immunodeficiency virus type 1 (HIV-1) Gag polyprotein is a critical determinant of virus assembly, and is therefore a potential target for developing drugs for AIDS therapy. Recently, a 12-mer α-helical peptide (CAI) was reported to disrupt immature- and mature-like capsid particle assembly in vitro; however, it failed to inhibit HIV-1 in cell culture due to its inability to penetrate cells. The same group reported the X-ray crystal structure of CAI in complex with the C-terminal domain of capsid (C-CA) at a resolution of 1.7 Å. Using this structural information, we have utilized a structure-based rational design approach to stabilize the α-helical structure of CAI and convert it to a cell-penetrating peptide (CPP). The modified peptide (NYAD-1) showed enhanced α-helicity. Experiments with laser scanning confocal microscopy indicated that NYAD-1 penetrated cells and colocalized with the Gag polyprotein during its trafficking to the plasma membrane where virus assembly takes place. NYAD-1 disrupted the assembly of both immature- and mature-like virus particles in cell-free and cell-based in vitro systems. NMR chemical shift perturbation analysis mapped the binding site of NYAD-1 to residues 169-191 of the C-terminal domain of HIV-1 capsid encompassing the hydrophobic cavity and the critical dimerization domain with an improved binding affinity over CAI. Furthermore, experimental data indicate that NYAD-1 most likely targets capsid at a post-entry stage. Most significantly, NYAD-1 inhibited a large panel of HIV-1 isolates in cell culture at low micromolar potency. Our study demonstrates how a structure-based rational design strategy can be used to convert a cell-impermeable peptide to a cell-permeable peptide that displays activity in cell-based assays without compromising its mechanism of action. This proof-of-concept cell-penetrating peptide may aid validation of capsid as an anti-HIV-1 drug target and may help in designing peptidomimetics and small molecule drugs targeted to this protein.     

Structure and localization of an essential transmembrane segment of the proton translocation channel of yeast H-V-ATPase

Vacuolar (H⁺)-ATPase (V-ATPase) is a proton pump present in several compartments of eukaryotic cells to regulate physiological processes. From biochemical studies it is known that the interaction between arginine 735 present in the seventh transmembrane (TM7) segment from subunit a and specific glutamic acid residues in the subunit c assembly plays an essential role in proton translocation. To provide more detailed structural information about this protein domain, a peptide resembling TM7 (denoted peptide MTM7) from Saccharomyces cerevisiae (yeast) V-ATPase was synthesized and dissolved in two membrane-mimicking solvents: DMSO and SDS. For the first time the secondary structure of the putative TM7 segment from subunit a is obtained by the combined use of CD and NMR spectroscopy. SDS micelles reveal an α-helical conformation for peptide MTM7 and in DMSO three α-helical regions are identified by 2D ¹H-NMR. Based on these conformational findings a new structural model is proposed for the putative TM7 in its natural environment. It is composed of 32 amino acid residues that span the membrane in an α-helical conformation. It starts at the cytoplasmic side at residue T719 and ends at the luminal side at residue W751. Both the luminal and cytoplasmatic regions of TM7 are stabilized by the neighboring hydrophobic transmembrane segments of subunit a and the subunit c assembly from V-ATPase.     

Structure of a Double Transmembrane Fragment of a G-Protein-Coupled Receptor in Micelles

The structure and dynamic properties of an 80-residue fragment of Ste2p, the G-protein-coupled receptor for α-factor of Saccharomyces cerevisiae, was studied in LPPG micelles with the use of solution NMR spectroscopy. The fragment Ste2p(G31-T110) (TM1-TM2) consisted of 19 residues from the N-terminal domain, the first TM helix (TM1), the first cytoplasmic loop, the second TM helix (TM2), and seven residues from the first extracellular loop. Multidimensional NMR experiments on [¹⁵N], [¹⁵N, ¹³C], [¹⁵N, ¹³C, ²H]-labeled TM1-TM2 and on protein fragments selectively labeled at specific amino acid residues or protonated at selected methyl groups resulted in >95% assignment of backbone and side-chain nuclei. The NMR investigation revealed the secondary structure of specific residues of TM1-TM2. TALOS constraints and NOE connectivities were used to calculate a structure for TM1-TM2 that was highlighted by the presence of three α-helices encompassing residues 39-47, 49-72, and 80-103, with higher flexibility around the internal Arg⁵⁸ site of TM1. RMSD values of individually superimposed helical segments 39-47, 49-72, and 80-103 were 0.25 ± 0.10 Å, 0.40 ± 0.13 Å, and 0.57 ± 0.19 Å, respectively. Several long-range interhelical connectivities supported the folding of TM1-TM2 into a tertiary structure typified by a crossed helix that splays apart toward the extracellular regions and contains considerable flexibility in the G⁵⁶VRSG⁶⁰ region. ¹⁵N-relaxation and hydrogen-deuterium exchange data support a stable fold for the TM parts of TM1-TM2, whereas the solvent-exposed segments are more flexible. The NMR structure is consistent with the results of biochemical experiments that identified the ligand-binding site within this region of the receptor.     

Crystal structure and characterization of coiled-coil domain of the transient receptor potential channel PKD2L1

The cation-permeable channel PKD2L1 forms a homomeric assembly as well as heteromeric associations with both PKD1 and PKD1L3, with the cytoplasmic regulatory domain (CRD) of PKD2L1 often playing a role in assembly and/or function. Our previous work indicated that the isolated PKD2L1 CRD assembles as a trimer in a manner dependent on the presence of a proposed oligomerization domain. Herein we describe the 2.7 Å crystal structure of a segment containing the PKD2L1 oligomerization domain which indicates that trimerization is driven by the β-branched residues at the first and fourth positions of a heptad repeat (commonly referred to as “a” and “d”) and by a conserved R-h-x-x-h-E salt bridge motif that is largely unique to parallel trimeric coiled coils. Further analysis of the PKD2L1 CRD indicates that trimeric association is sufficiently strong that no other species are present in solution in an analytical ultracentrifugation experiment at the lowest measurable concentration of 750 nM. Conversely, mutation of the “a” and “d” residues leads to formation of an exclusively monomeric species, independent of concentration. Although both monomeric and WT CRDs are stable in solution and bind calcium with 0.9 μM affinity, circular dichroism studies reveal that the monomer loses 25% more α-helical content than WT when stripped of this ligand, suggesting that the CRD structure is stabilized by trimerization in the ligand-free state. This stability could play a role in the function of the full-length complex, indicating that trimerization may be important for both homo- and possibly heteromeric assemblies of PKD2L1.     

Crystal structure studies of NADP dependent isocitrate dehydrogenase from Thermus thermophilus exhibiting a novel terminal domain

NADP⁺ dependent isocitrate dehydrogenase (IDH) is an enzyme catalyzing oxidative decarboxylation of isocitrate into oxalosuccinate (intermediate) and finally the product α-ketoglutarate. The crystal structure of Thermus thermophilus isocitrate dehydrogenase (TtIDH) ternary complex with citrate and cofactor NADP⁺ was determined using X-ray diffraction method to a resolution of 1.80 Å. The overall fold of this protein was resolved into large domain, small domain and a clasp domain. The monomeric structure reveals a novel terminal domain involved in dimerization, very unique and novel domain when compared to other IDH’s. And, small domain and clasp domain showing significant differences when compared to other IDH’s of the same sub-family. The structure of TtIDH reveals the absence of helix at the clasp domain, which is mainly involved in oligomerization in other IDH’s. Also, helices/beta sheets are absent in the small domain, when compared to other IDH’s of the same sub family. The overall TtIDH structure exhibits closed conformation with catalytic triad residues, Tyr144-Asp248-Lys191 are conserved. Oligomerization of the protein is quantized using interface area and subunit–subunit interactions between protomers. Overall, the TtIDH structure with novel terminal domain may be categorized as a first structure of subfamily of type IV.     

Residues in a conserved α-helical segment are required for cleavage but not secretion of an Escherichia coli serine protease autotransporter passenger domain

Autotransporters are a superfamily of virulence factors produced by Gram-negative bacteria that are comprised of an N-terminal extracellular domain (passenger domain) and a C-terminal β barrel domain (β domain) that resides in the outer membrane (OM). The β domain promotes the translocation of the passenger domain across the OM by an unknown mechanism. Available evidence indicates that an α-helical segment that spans the passenger domain-β domain junction is embedded inside the β domain at an early stage of assembly. Following its secretion, the passenger domain of the serine protease autotransporters of the Enterobacteriaceae (SPATEs) and the pertactin family of Bordetella pertussis autotransporters is released from the β domain through an intrabarrel autoproteolytic cleavage of the α-helical segment. Although the mutation of conserved residues that surround the cleavage site has been reported to impair both the translocation and cleavage of the passenger domain of a SPATE called Tsh, we show here that the mutation of the same residues in another SPATE (EspP) affects only passenger domain cleavage. Our results strongly suggest that the conserved residues are required to position the α-helical segment for the cleavage reaction and are not required to promote passenger domain secretion.     

Structure and mode of action of a mosquitocidal holotoxin

The crystal structure of the full mosquitocidal toxin from Bacillus sphaericus (MTX_holo) has been determined at 2.5 Å resolution by the molecular replacement method. The resulting structure revealed essentially the complete chain consisting of four ricin B-type domains curling around the catalytic domain in a hedgehog-like assembly. As the structure was virtually identical in three different crystal packings, it is probably not affected by packing contacts. The structure of MTX_holo explains earlier autoinhibition data. An analysis of published complexes comprising ricin B-type lectin domains and sugar molecules shows that the general construction principle applies to all four lectin domains of MTX_holo, indicating 12 putative sugar-binding sites. These sites are sequence-related to those of the cytotoxin pierisin from cabbage butterfly, which are known to bind glycolipids. It seems therefore likely that MTX_holo also binds glycolipids. The seven contact interfaces between the five domains are predominantly polar and not stronger than common crystal contacts so that in an appropriate environment, the multidomain structure would likely uncurl into a string of single domains. The structure of the isolated catalytic domain plus an extended linker was established earlier in three crystal packings, two of which showed a peculiar association around a 7-fold axis. The catalytic domain of the reported MTX_holo closely resembles all three published structures, except one with an appreciable deviation of the 40 N-terminal residues. A comparison of all structures suggests a possible scenario for the translocation of the toxin into the cytosol.     

Ca-ATPase structure in the E1 and E2 conformations: mechanism, helix-helix and helix-lipid interactions

The determination of the crystal structure of the Ca²⁺-ATPase of sarcoplasmic reticulum (SR) in its Ca²⁺-bound [Nature 405 (2000) 647] and Ca²⁺-free forms [Nature 418 (2002) 605] gives the opportunity for an analysis of conformational changes on the Ca²⁺-ATPase and of helix-helix and helix-lipid interactions in the transmembrane (TM) region of the ATPase. The locations of the ends of the TM α-helices on the cytoplasmic side of the membrane are reasonably well defined by the location of Trp residues and by the location of Lys-262 that snorkels up to the surface. The locations of the lumenal ends of the helices are less clear. The position of Lys-972 on the lumenal side of helix M9 suggests that the hydrophobic thickness of the protein is only about 21 Å, rather than the normal 30 Å. The experimentally determined TM α-helices do not agree well with those predicted theoretically. Charged headgroups are required for strong interaction of lipids with the ATPase, consistent with the large number of charged residues located close to the lipid-water interface. Helix packing appears to be rather irregular. Packing of helices M8 and M10 is of the 3-4 ridges-into-grooves or knobs-into-holes types. Packing of helices M5 and M7 involves two Gly residues in M7 and one Gly residue in M5. Packing of the other helices generally involves just one or two residues on each helix at the crossing point. The irregular packing of the TM α-helices in the Ca²⁺-ATPase, combined with the diffuse structure of the ATPase on the lumenal side of the membrane, is suggested to lead to a relative low activation energy for changing the packing of the TM α-helices, with changes in TM α-helical packing being important in the process of transfer of Ca²⁺ ions across the membrane. The inhibitor thapsigargin binds in a cleft between TM α-helices M3, M5 and M7. It is suggested that this and other similar clefts provide binding sites for a variety of hydrophobic molecules affecting the activity of the Ca²⁺-ATPase.     

Structural basis of poly(3-hydroxybutyrate) hydrolysis by PhaZ7 depolymerase from Paucimonas lemoignei

The crystal structure of poly(3-hydroxybutyrate) (PHB) depolymerase PhaZ7 purified from Paucimonas lemoignei was determined at 1.90 Å resolution. The structure consists of a single domain with an α/β hydrolase fold in its core. The active site is analogous to that of serine esterases/lipases and is characterized by the presence of a catalytic triad comprising Ser136, Asp242, and His306. Comparison with other structures in the Protein Data Bank showed a high level of similarity with the Bacillus subtilis lipase LipA (RMSD, 1.55 Å). Structural comparison with Penicillium funiculosum PHB depolymerase, the only PHB depolymerase whose structure is already known, revealed significant differences, resulting in an RMSD of 2.80-3.58 Å. The two enzymes appear to utilize different types of solvent-exposed residues for biopolymer binding, with aliphatic and hydroxyl residues used in P. funiculosum PHB depolymerase and aromatic residues in PhaZ7. Moreover, the active site of P. funiculosum PHB depolymerase is accessible to the substrate in contrast to the active site of PhaZ7, which is buried. Hence, considerable conformational changes are required in PhaZ7 for the creation of a channel leading to the active site. Taken together, the structural data suggest that PhaZ7 and P. funiculosum PHB depolymerase have adopted different strategies for effective substrate binding in response to their diverse substrate specificity and the lack of a substrate-binding domain.     

A novel thermostable membrane protease forming an operon with a stomatin homolog from the hyperthermophilic archaebacterium Pyrococcus horikoshii

Membrane-bound proteases play several important roles in protein quality control and regulation. In the genome of the hyperthermophilic archaebacterium Pyrococcus horikoshii, the open reading frames PH1510 and PH1511 are homologous to the genes nfed (nodulation formation efficiency D) and stomatin, respectively, and probably form an operon. The nfed proteins are putative membrane proteins, and the N-terminal region shows homology to ClpP-type serine proteases. Stomatin is one of the major integral membrane proteins of human erythrocytes, and its absence is associated with the hemolytic anemia known as hereditary stomatocytosis. Thus, the N-terminal region of PH1510 (1510-N, residues 16-236) was expressed and purified. From activity staining and SDS-PAGE analysis using fluorescein isothiocyanate-casein, 1510-N was identified as a thermostable endo-type protease. From site-directed mutagenesis, the conserved Ser-97 and Lys-138 are involved in proteolysis and, therefore, PH1510 is probably a serine protease with a catalytic Ser-Lys dyad. The sites of cleavage by 1510-N are rich in hydrophobic residues. The site P1 (position -1 relative to the cleavage site) is mainly leucine. P4 and P4' are mainly hydrophobic residues. Interestingly, the 1510-N protease cleaves the C-terminal hydrophobic region of PH1511. From this result and the probability of an operon, PH1510 probably functions in cooperation with PH1511. It is hypothesized that the cleavage of the stomatin-homolog PH1511 by the PH1510 protease causes an ion channel to open.