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J Biol Chem. 2014 Apr 18;289(16):11262-71. doi: 10.1074/jbc.M113.521773. Epub 2014 Mar 4.

A disease-causing mutation illuminates the protein membrane topology of the kidney-expressed prohibitin homology (PHB) domain protein podocin.

Author information

1
From the Department II of Internal Medicine and Center for Molecular Medicine Cologne, University of Cologne, Cologne, Germany.

Abstract

Mutations in the NPHS2 gene are a major cause of steroid-resistant nephrotic syndrome, a severe human kidney disorder. The NPHS2 gene product podocin is a key component of the slit diaphragm cell junction at the kidney filtration barrier and part of a multiprotein-lipid supercomplex. A similar complex with the podocin ortholog MEC-2 is required for touch sensation in Caenorhabditis elegans. Although podocin and MEC-2 are membrane-associated proteins with a predicted hairpin-like structure and amino and carboxyl termini facing the cytoplasm, this membrane topology has not been convincingly confirmed. One particular mutation that causes kidney disease in humans (podocin(P118L)) has also been identified in C. elegans in genetic screens for touch insensitivity (MEC-2(P134S)). Here we show that both mutant proteins, in contrast to the wild-type variants, are N-glycosylated because of the fact that the mutant C termini project extracellularly. Podocin(P118L) and MEC-2(P134S) did not fractionate in detergent-resistant membrane domains. Moreover, mutant podocin failed to activate the ion channel TRPC6, which is part of the multiprotein-lipid supercomplex, indicative of the fact that cholesterol recruitment to the ion channels, an intrinsic function of both proteins, requires C termini facing the cytoplasmic leaflet of the plasma membrane. Taken together, this study demonstrates that the carboxyl terminus of podocin/MEC-2 has to be placed at the inner leaflet of the plasma membrane to mediate cholesterol binding and contribute to ion channel activity, a prerequisite for mechanosensation and the integrity of the kidney filtration barrier.

KEYWORDS:

Caenorhabditis elegans; Cholesterol-binding Protein; Glycosylation; Lipid Raft; Membrane Topology; PHB Domain; Podocin; Protein Conformation; Steroid-resistant Nephrotic Syndrome; Stomatin Family Proteins

PMID:
24596097
PMCID:
PMC4036264
DOI:
10.1074/jbc.M113.521773
[Indexed for MEDLINE]
Free PMC Article

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