The dimeric ancillary β subunits Ostm1 surround the CLC-7 dimer at the TMD and the intravesicular regions ( Fig. Each monomer contains two cystathionine-β-synthase subdomains (CBS1 and CBS2), which are tightly packed against each other through a pseudo-twofold symmetry axis ( Fig. The transmembrane region of the complex consists of two independent hourglass-shaped transport pathways constricted by selectivity filters ( Fig. In agreement with previously resolved CLC proteins ( 11– 15), CLC-7 adopts a dimeric architecture with each monomer consisting of 24 α helices and 5 β strands ( Fig. It is thus clear that further studies are required to elucidate the physiological role of CLC-7 transporter. In addition, a study reporting the identification of a gain-of-function mutation (Y715C) in CLC-7 that alters lysosomal pH ( 24) provides additional evidence to support a functional role for CLC-7 in regulating lysosomal pH. However, later studies established that lysosomal pH is normal in Clcn7 −/− and Ostm1 −/− mice ( 18, 19, 23), suggesting that the major role of CLC-7 is to increase the luminal Cl − concentration by exploiting the proton gradient created by H +-ATPase, rather than providing a shunting conductance. Initially, CLC-7 was suggested to provide a means for charge-compensating conductance for proton pumping, thereby facilitating lysosomal acidification ( 22). The specific H + concentration in the lysosome is mainly achieved through proton pump V-type H +-dependent adenosine triphosphatases (H +-ATPases) ( 21), and the physiological role of CLC-7 in lysosomal acidification remains controversial ( 1). Accumulating evidence suggests that Ostm1 serves as an ancillary β subunit of CLC-7 to support bone resorption and lysosomal function ( 7, 19). The detailed mechanism underlying the slow gating process remains mysterious.ĬLC-7 functions as an electrogenic antiporter that mainly resides in lysosomes and osteoclast ruffled membranes ( 18– 20). Previous macroscopic current and single-channel studies have indicated that CLC proteins apparently display two distinct types of gating process ( 9, 16, 17): fast gating (“protopore gate”), wherein the ion-conducting pore of one subunit opens and closes independently of the other subunit and which is thought to result largely from a small movement of the gate glutamate residue positioned within the ion-conducting pore, and slow gating (“common gate”), which operates both pores simultaneously. CLC proteins function as dimers, with each subunit having its own passageway for ion transport ( 9– 15). The remaining CLC proteins (CLC-3 to CLC-7) are electrogenic Cl −/H + antiporters ( 5– 8) that exchange Cl − and H + with a stoichiometry of 2:1. The first subtype consists of Cl − channels: CLC-1, CLC-2, CLC-Ka, and CLC-Kb, which are mainly found at the cell membrane, where they function to control the Cl − flow and to stabilize the membrane potential ( 4). Human CLC proteins can be divided into two subtypes. The CLC family comprises a group of integral membrane proteins that translocate Cl − across the cell membranes members of this family are essential for the maintenance of membrane potential, regulation of transepithelial Cl − transport, and control of intravesicular pH ( 1– 3). Thus, our study deepens understanding of CLC-7/Ostm1 transporter and provides insights into the molecular basis of the disease-related mutations. Structural analyses and electrophysiology studies suggest that the domain interaction interfaces affect the slow gating kinetics of CLC-7/Ostm1. Our complex structure reveals a functionally crucial domain interface between the amino terminus, TMD, and CBS domains of CLC-7. Here, we present the cryo–electron microscopy (cryo-EM) structure of the human CLC-7/Ostm1 complex and reveal that the highly glycosylated Ostm1 functions like a lid positioned above CLC-7 and interacts extensively with CLC-7 within the membrane. Mutations in human CLC-7/Ostm1 lead to lysosomal storage disorders and severe osteopetrosis. CLC-7/Ostm1 is an electrogenic Cl −/H + antiporter that mainly resides in lysosomes and osteoclast ruffled membranes.
CLC family proteins translocate chloride ions across cell membranes to maintain the membrane potential, regulate the transepithelial Cl − transport, and control the intravesicular pH among different organelles.
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