The prohibitin (PHB)-domain proteins are membrane proteins that regulate a variety of biological activities, including mechanosensation, osmotic homeostasis, and cell signaling, although the mechanism of this regulation is unknown. We have studied two members of this large protein family, MEC-2, which is needed for touch sensitivity in Caenorhabditis elegans, and Podocin, a protein involved in the function of the filtration barrier in the mammalian kidney, and find that both proteins bind cholesterol. This binding requires the PHB domain (including palmitoylation sites within it) and part of the N-terminally adjacent hydrophobic domain that attaches the proteins to the inner leaflet of the plasma membrane. By binding to MEC-2 and Podocin, cholesterol associates with ion-channel complexes to which these proteins bind: DEG͞ENaC channels for MEC-2 and TRPC channels for Podocin. Both the MEC-2-dependent activation of mechanosensation and the Podocin-dependent activation of TRPC channels require cholesterol. Thus, MEC-2, Podocin, and probably many other PHB-domain proteins by binding to themselves, cholesterol, and target proteins regulate the formation and function of large protein-cholesterol supercomplexes in the plasma membrane.prohibitin-domain proteins ͉ TRP channels ͉ DEG/ENaC channels ͉ slit diaphragm ͉ mechanosensation T he prohibitin homology (PHB)-domain proteins constitute a family of Ϸ1,800 proteins (SMART database; http:͞͞smart. embl-heidelberg.de) (1) all of which share an Ϸ150-aa domain similar to that in the mitochondrial protein prohibitin (2). More than 340 of these proteins, many of which have an N-terminal adjacent hydrophobic region that places them on the inner leaflet of the lipid bilayer, have been identified in animals. These membrane-associated proteins regulate osmotic homeostasis, mechanosensation, and cell signaling (3-5). Several of the animal PHB-domain proteins including flotillin, Podocin, prohibitin, stomatin, UNC-1, UNC-24, and the UNC-24-like mammalian protein SLP-1 are found in cholesterol-rich membrane fractions derived from the plasma membrane (reviewed in ref.2).In this article, we investigate the function of these proteins using two members of the family, MEC-2 from Caenorhabditis elegans and Podocin from mouse. MEC-2 (6) and Podocin (7) have a single, central hydrophobic domain that embeds these proteins in the inner leaflet of the plasma membrane with their N-and C-terminal tails facing the cytoplasm (Fig. 1a). Although the two proteins contain different N and C termini, they have hydrophobic regions that are 35% identical and 75% similar and PHB-domains that are 50% identical and 80% similar (Fig. 1b). The PHB domain is critical for the action of both proteins (8, 9).

We found that MAGED2 mutations caused X-linked polyhydramnios with prematurity and a severe but transient form of antenatal Bartter's syndrome. MAGE-D2 is essential for fetal renal salt reabsorption, amniotic fluid homeostasis, and the maintenance of pregnancy. (Funded by the University of Groningen and others.).

Cilia are specialized organelles that play an important role in several biological processes, including mechanosensation, photoperception, and osmosignaling. Mutations in proteins localized to cilia have been implicated in a growing number of human diseases. In this study, we demonstrate that the von Hippel-Lindau (VHL) protein (pVHL) is a ciliary protein that controls ciliogenesis in kidney cells. Knockdown of pVHL impeded the formation of cilia in mouse inner medullary collecting duct 3 kidney cells, whereas the expression of pVHL in VHL-negative renal cancer cells rescued the ciliogenesis defect. Using green fluorescent protein–tagged end-binding protein 1 to label microtubule plus ends, we found that pVHL does not affect the microtubule growth rate but is needed to orient the growth of microtubules toward the cell periphery, a prerequisite for the formation of cilia. Furthermore, pVHL interacts with the Par3–Par6–atypical PKC complex, suggesting a mechanism for linking polarity pathways to microtubule capture and ciliogenesis.

The cilia-associated protein NPHP4 is a negative regulator of Hippo signaling that modulates cell proliferation in mammals.

The kidney filter represents a unique assembly of podocyte epithelial cells that tightly enwrap the glomerular capillaries with their foot processes and the interposed slit diaphragm. So far, very little is known about the guidance cues and polarity signals required to regulate proper development and maintenance of the glomerular filtration barrier. We now identify Par3, Par6, and atypical protein kinase C (aPKC) polarity proteins as novel Neph1-Nephrin-associated proteins. The interaction was mediated through the PDZ domain of Par3 and conserved carboxyl terminal residues in Neph1 and Nephrin. Par3, Par6, and aPKC localized to the slit diaphragm as shown in immunofluorescence and immunoelectron microscopy. Consistent with a critical role for aPKC activity in podocytes, inhibition of glomerular aPKC activity with a pseudosubstrate inhibitor resulted in a loss of regular podocyte foot process architecture. These data provide an important link between cell recognition mediated through the Neph1-Nephrin complex and Par-dependent polarity signaling and suggest that this molecular interaction is essential for establishing the three-dimensional architecture of podocytes at the kidney filtration barrier.