8.A.21 The Stomatin/Podocin/Band 7/Nephrosis.2/SPFH (Stomatin) Family
Stomatin 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).
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).