8.A.21 The Stomatin/Podocin/Band 7/Nephrosis.2/SPFH (Stomatin) Family
Stomatin (STOM) is one of the major integral membrane proteins of the human erythrocyte (Band 7.2b), and its absence is associated with the hemolytic anemia condition known as hereditary stomatocytosis (hydrocytosis). Stomatin is thought to function as a negative regulator of univalent cation permeability. Its homologues are found in almost all species of eukaryotes, bacteria and archaea. In many prokaryotes the stomatin-encoding genes are in bicistronic operons that also encode integral membrane proteases with one N-terminal TMS, an N-terminal ClpP-type serine endoprotease domain, and a C-terminal 6 TMS hydrophobic domain. The proteases cleave the C-terminal hydrophobic regions in the stomatin homologue (Yokoyama and Matsui, 2005). The cleavage of the stomatin homologue by the protease may cause an ion channel to open. The erythrocyte stomatin may have 3 TMSs, one very hydrophobic TMS (residues 27-51), and two moderately hydrophobic TMSs (residues 78-94 and 265-282) in this 288 aa protein. The Mec2 protein of C. elegans is a subunit in the touch responsive mechanosensitive degenerin channel complex in the ENaC family (TC #1.A.6.2.2). Stomatins are monotopic integral membrane proteins found in all classes of life that regulate members of the acid-sensing ion channel (ASIC) family (TC# 1.A.6). Regulation requires two distinct sites on ASIC3: the distal C-terminus and the first TMS1. The C-terminal site is critical for formation of the STOM-ASIC3 complex, while TMS1 is required only for the regulatory effect (Klipp et al. 2020).
Human erythrocytes express the highest level of the Glut1 glucose transporter (TC# 2.A.1.1.28). Glucose transport decreases during human erythropoiesis despite a >3-log increase in Glut1 transcripts. Glut1-mediated transport of L-dehydroascorbic acid (DHA), an oxidized form of ascorbic acid (AA), is dramatically enhanced. Stomatin, regulates the switch from glucose to DHA transport (Montel-Hagen et al., 2008). Erythrocyte Glut1 and associated DHA uptake are unique traits of humans and the few other mammals that have lost the ability to synthesize AA from glucose. Mice, a species capable of synthesizing AA, express Glut4 but not Glut1 in mature erythrocytes. Thus, erythrocyte-specific coexpression of Glut1 with stomatin constitutes a compensatory mechanism in mammals that are unable to synthesize vitamin C.
Nephrotic syndrome (NS) is manifested by hyperproteinuria, hypoalbuminemia, and edema. The NPHS2 gene that encodes podocin has the most mutations among the genes that are involved in the pathophysiology of NS. Podocin is expressed exclusively in podocytes and is localized to the slit-diaphragm (SD). Mutations in podocin are associated with steroid-resistant NS and rapid progression to end-stage renal disease, thus signifying its role in maintaining SD integrity. Mulukala et al. 2016 deduced a model for human podocin, discussed the details of transmembrane localization and intrinsically unstrucMembrane fusions that occur during vesicle transport, virus infection, and tissue development, involve receptors that mediate membrane contact and initiate fusion and effectors that execute membrane reorganization and fusion pore formation. Some of these ftured regions, and provided an understanding of how podocin interacts with other SD components.
Some fusogenic receptors/effectors are preferentially recruited to lipid raft membrane microdomains, and stomatin is a major constituents of lipid rafts (Lee et al. 2016). Cells expressing more stomatin or exposed to exogenous stomatin are more prone to undergoing cell fusion. During osteoclastogenesis, depletion of stomatin inhibits cell fusion, and in stomatin transgenic mice, increased cell fusion leading to enhanced bone resorption and subsequent osteoporosis were observed. With its unique molecular topology, stomatin forms molecular assembly within lipid rafts or on the appositional plasma membranes, and promotes membrane fusion by modulating fusogenic protein engagement (Lee et al. 2016).
Bacteria have homologues that appear to play roles in membrane stress adaptation (Akiyama 2009).
References:
Erythrocyte stomatin (STOM; Band 7) (Similar to Mechanosensory protein Mec2, a stomatin-like subunit of the ASIC channel with TC# 1.A.6.2.2). STOM is an inhibitor of ASIC3 (TC# 1.A.6.1.2), and is anchored to the ASIC3 channel via a site on the distal C-terminus of the channel. This interaction stabilizes the desensitized state via an interaction with TMS1 in ASIC3 (Klipp et al. 2020).
Animals
Stomatin of Homo sapiens (P27105)
Podocin of 383 aas and 1 TMS. Expressed exclusively in the slit-diaphragm of podocytes. Mutations lead to steroid resistance followed by renal disease (Mulukala et al. 2016). Podocytes are crucial cells of the glomerular filtration unit that play a vital role at the interface of the blood-urine barrier. Podocyte slit-diaphragm is a modified tight junction that facilitates size and charge-dependent permselectivity. Several proteins including podocin, nephrin (TC# 8.a.23.1.33), CD2AP (8.A.34.1.5), and TRPC6 (1.A.4.1.5) form a macromolecular assembly that constitutes the slit-diaphragm that resembles tight junctions. Podocin is an integral membrane protein attached to the inner membrane of the podocyte via a short transmembrane region (101-125). The cytosolic N- and C-termini help podocin attain a hook-like structure. The protein forms a homooligomer, a 16-mer (Mulukala et al. 2020).
Podocin of Homo sapiens
Stomatin-like protein 3, STROML3 of 291 aas and 2 TMSs. Functions in the regulation of ASIC channels in mobile vesicles in sensory neurons (Lapatsina et al. 2012).
STROML3 of Homo sapiens
Protease modulator HflC of 287 aas and 1 N-terminal TMS.
HflC of sulfur-oxidizing endosymbiont of Gigantopelta aegis
Stomatin homologue. Cleavage of this protein by a protease encoded within the same operon as the stomatin has been reported to open an ion channel (Yokoyama and Matsui, 2005).
Prokaryotes
Stomatin homologue and its protease of Pyrococcus horikoshii
Stomatin homologue (O59180)
Protease (NP_143370)
Membrane protease subunit, stomatin/prohibitin homolog, NCgl2533 of 432 aas and 1 strongly hydrophobic N-terminal TMS followed immediately by three moderately hydrophobic potential TMSs. Expression of NCgl2533 increased the alanine concentration in cell culture (Nanatani et al. 2022).
NCgl2533 of Corynebacterium glutamicum
QmcA protein (bacterial homologue) of 305 aas and 1 TMS (N-terminal). May play a role in the quality control of integral membrane proteins. There is no evidence that it plays a role in transport.
QmcA of E. coli
Prohibitin, Wph or Phb, of273 aas and 1 N-terminal TMS.
Prohibitin of Triticum aestivum (Wheat)
Signal peptide peptidase of 325 aas and 4 or 5 TMSs, SppA1.
Peptidase of Bdellovibrio exovorus
Modulator of FtsH protease, HflK, of 266 aas and 1 strong N-terminal TMS as well as as many as 3 or 4 additional less hydrophobic peaks that could be TMSs. It shows strong similarity to recognized stomatin proteins and may function with an uncharacterized protein of 231 aas and 6 TMSs in a 2 + 2 + 2 TMS arrangement encoded by the gene adjacent to the hflK gene. This latter uncharacterized protein exhibits weak similarity to the second half of the NfeD protease (TC# 8.A.21.2.1). The protease domain of NfeD is in the first half.
HflK/ORF of Candidatus Thorarchaeota archaeon AB_25
Membrane protease of 289 aas and 2 or 3 N-terminal TMSs of the stomatin/prohibitin family. The gene encoding this protein is adjacent to the two genes encoding an ABC exporter with TC# 3.A.1.143.4.
Protease of Thermobacillus composti KWC4
Signal peptide peptidase SppA (protease IV in the chromatophore of Paulinella chromatophora
of 276 aas.
SppA of Paulinella chromatophora
Prohibitin, Phb, of 272 aas and 1 N-terminal TMS. It plays a role in glucose homeostasis in adipose tissue. Expression of its structural gene is affected by emodin which also decreases body weight and blood lipids while increasing glucose tolerance and ceramides (Cheng et al. 2021). Together with PHB2, it forms large ring complexes (prohibitin complexes) in the inner mitochondrial membrane (IMM) and functions as a chaperone protein that stabilizes mitochondrial respiratory enzymes and maintains IMM mitochondrial integrity. It regulates mitochondrial respiratory activity, playing a role in cellular aging (Coates et al. 2001). Bakuchiol targets prohibitins and mitochondrial proteins including VDACs (Shoji et al. 2024).
Phb of Homo sapiens
Hypersensitive induced response protein 3, HIR3, of 287 aas and 1 N-terminal TMS. It was isolated from wheat infected by stripe rust fungus. It contains the SPFH (Stomatins, Prohibitins, Flotillins and HflK/C) protein domain typical of stomatins which serve as a negative regulators of univalent cation permeability, especially for potassium (Yu et al. 2008).
HIR3 of Triticum aestivum (Wheat)
Flotillin-1 (FLOT1) or Reggie 2 (Reg2) of 423 aas and 1 N-terminal TMS. Flotillin upregulation is necessary and sufficient to promote epithelial and mesenchymal cancer cell invasion and ECM degradation by controlling endocytosis and delivery of the transmembrane protease, MT1-MMP or MMP14 (TC# 8.B.14.2.3) to the endolysosomal recycling compartment (Planchon et al. 2018). Flotillin-2, a paralog of Flotillin-1 (48% identity) (see TC# 8.A.21.3.2), serves the same or an overlapping function. Flotillin-1 interacts with and sustains the surface levels of the TRPV2 channel (Hu et al. 2021). REG2 regulates Ca2+ influx and insulin secretion in pancreatic islets. It forms a cascade of glutathione peroxidase-1 (Gpx1)/REG2/CaV1.2 (TC# 1.A.1.11.4) to explain how REG2 depletion in overexpressing islets decreases its binding to CaV1.2, resulting in uninhibited Ca2+ influx and augmented GSIS. This bridges redox biology, tissue regeneration, and insulin secretion (Yan et al. 2022).
Flotillin-1 or REG2 of Homo sapiens
Flotillin-2 (Flot-2 or Flot2) or Reggie 1 (Reg-1 or Reg1), of 428 aas and 1 N-terminal TMS. It may play a role in axon growth and regeneration as well as in epidermal cell adhesion and epidermal structure and function (Lang et al. 1998).
Flot-2 of Rattus norvegicus (Rat)