The Caenorhabditis elegans behavioral gene unc-24 encodes a novel bipartite protein similar to both erythrocyte band 7.2 (stomatin) and nonspecific lipid transfer protein

Exposure to dexamethasone modifies transcriptomic responses of free-living stages of Strongyloides stercoralis

Hyperinfection and disseminated infection by the parasitic nematode Strongyloides stercoralis can be induced by iatrogenic administration of steroids and immunosuppression and lead to an elevated risk of mortality. Responses of free-living stages of S. stercoralis to the therapeutic corticosteroid dexamethasone (DXM) were investigated using RNA-seq transcriptomes of DXM-treated female and male worms. A total of 17,950 genes representing the transcriptome of these free-living adult stages were obtained, among which 199 and 263 were differentially expressed between DXM-treated females and DXM-treated males, respectively, compared with controls. According to Gene Ontology analysis, differentially expressed genes from DXM-treated females participate in developmental process, multicellular organismal process, cell differentiation, carbohydrate metabolic process and embryonic morphogenesis. Others are involved in signaling and signal transduction, including cAMP, cGMP-dependent protein kinase pathway, endocrine system, and thyroid hormone pathway, as based on Kyoto Encyclopedia of Genes and Genomes analysis. The novel findings warrant deeper investigation of the influence of DXM on growth and other pathways in this neglected tropical disease pathogen, particularly in a setting of autoimmune and/or allergic disease, which may require the clinical use of steroid-like hormones during latent or covert strongyloidiasis.

Adhesion GPCRs as a Putative Class of Metabotropic Mechanosensors

Adhesion GPCRs as mechanosensors. Different aGPCR homologs and their cognate ligands have been described in settings, which suggest that they function in a mechanosensory capacity. For details, see text G protein-coupled receptors (GPCRs) constitute the most versatile superfamily of biosensors. This group of receptors is formed by hundreds of GPCRs, each of which is tuned to the perception of a specific set of stimuli a cell may encounter emanating from the outside world or from internal sources. Most GPCRs are receptive for chemical compounds such as peptides, proteins, lipids, nucleotides, sugars, and other organic compounds, and this capacity is utilized in several sensory organs to initiate visual, olfactory, gustatory, or endocrine signals. In contrast, GPCRs have only anecdotally been implicated in the perception of mechanical stimuli. Recent studies, however, show that the family of adhesion GPCRs (aGPCRs), which represents a large panel of over 30 homologs within the GPCR superfamily, displays molecular design and expression patterns that are compatible with receptivity toward mechanical cues (Fig. 1). Here, we review physiological and molecular principles of established mechanosensors, discuss their relevance for current research of the mechanosensory function of aGPCRs, and survey the current state of knowledge on aGPCRs as mechanosensing molecules.

Structural and functional characterization of a novel gene, Hc-daf-22, from the strongylid nematode Haemonchus contortus

BACKGROUND

The strongylid nematode Haemonchus contortus is a parasite of major concern for modern livestock husbandry because hostile environmental conditions may induce diapause in the early fourth-stage larvae.

METHODS

A new gene Hc-daf-22 was identified which is the homologue of Ce-daf-22 and human SCPx. Genome walking and RACE were performed to obtain the whole cDNA and genomic sequence of this gene. Using qRT-PCR with all developmental stages as templates to explore the transcription level and micro-injection was applied to confirm the promoter activity of the 5'-flanking region. Overexpression, rescue and RNA interference experiments were performed in N2, daf-22 mutant (ok 693) strains of C. elegans to study the gene function of Hc-daf-22.

RESULTS

The full length gene of Hc-daf-22 (6,939 bp) contained 16 exons separated by 15 introns, and encoded a cDNA of 1,602 bp (533 amino acids, estimated at about 59.3 kDa) with a peak in L3 and L4 in transcriptional level. The Hc-DAF-22 protein was consisted of a 3-oxoacyl-CoA thiolase domain and a SCP2 domain and evolutionarily conserved. The 1,548 bp fragment upstream of the 5'-flanking region was confirmed to have promoter activity compared with 5'-flanking region of Ce-daf-22. The rescue experiment by micro-injection of daf-22 (ok693) mutant strain showed significant increase in body size and brood size in the rescued worms with significantly reduced or completely absent fat granules confirmed by Oil red O staining, indicating that Hc-daf-22 could partially rescue the function of Ce-daf-22. Furthermore, RNAi with Hc-daf-22 could partially silence the endogenous Ce-daf-22 in N2 worms and mimic the phenotype of daf-22 (ok693) mutants.

CONCLUSION

The gene Hc-daf-22 was isolated and its function identified using C. elegans as a model organism. Our results indicate that Hc-daf-22 shared similar characteristics and function with Ce-daf-22 and may play an important role in peroxisomal β-oxidation and the development in H. contortus.

Subunit-specific inhibition of acid sensing ion channels by stomatin-like protein 1

There are five mammalian stomatin-domain genes, all of which encode peripheral membrane proteins that can modulate ion channel function. Here we examined the ability of stomatin-like protein 1 (STOML1) to modulate the proton-sensitive members of the acid-sensing ion channel (ASIC) family. STOML1 profoundly inhibits ASIC1a, but has no effect on the splice variant ASIC1b. The inactivation time constant of ASIC3 is also accelerated by STOML1. We examined STOML1 null mutant mice with a β-galactosidase-neomycin cassette gene-trap reporter driven from the STOML1 gene locus, which indicated that STOML1 is expressed in at least 50% of dorsal root ganglion (DRG) neurones. Patch clamp recordings from mouse DRG neurones identified a trend for larger proton-gated currents in neurones lacking STOML1, which was due to a contribution of effects upon both transient and sustained currents, at different pH, a finding consistent with an endogenous inhibitory function for STOML1.

The atypical calpains: evolutionary analyses and roles in Caenorhabditis elegans cellular degeneration

The calpains are physiologically important Ca²⁺-activated regulatory proteases, which are divided into typical or atypical sub-families based on constituent domains. Both sub-families are present in mammals, but our understanding of calpain function is based primarily on typical sub-family members. Here, we take advantage of the model organism Caenorhabditis elegans, which expresses only atypical calpains, to extend our knowledge of the phylogenetic evolution and function of calpains. We provide evidence that a typical human calpain protein with a penta EF hand, detected using custom profile hidden Markov models, is conserved in ancient metazoans and a divergent clade. These analyses also provide evidence for the lineage-specific loss of typical calpain genes in C. elegans and Ciona, and they reveal that many calpain-like genes lack an intact catalytic triad. Given the association between the dysregulation of typical calpains and human degenerative pathologies, we explored the phenotypes, expression profiles, and consequences of inappropriate reduction or activation of C. elegans atypical calpains. These studies show that the atypical calpain gene, clp-1, contributes to muscle degeneration and reveal that clp-1 activity is sensitive to genetic manipulation of [Ca²⁺]_i. We show that CLP-1 localizes to sarcomeric sub-structures, but is excluded from dense bodies (Z-disks). We find that the muscle degeneration observed in a C. elegans model of dystrophin-based muscular dystrophy can be suppressed by clp-1 inactivation and that nemadipine-A inhibition of the EGL-19 calcium channel reveals that Ca²⁺ dysfunction underlies the C. elegans MyoD model of myopathy. Taken together, our analyses highlight the roles of calcium dysregulation and CLP-1 in muscle myopathies and suggest that the atypical calpains could retain conserved roles in myofilament turnover.

Calpains are calcium activated non-lysosomal proteases that cleave proteins with exquisite selectivity. Proteins can be activated by calpain cleavage, because they are released from inhibitory constraints, or they can be targeted for further degradation to facilitate their normal physiological turnover or to promote cellular remodelling. Inappropriate calpain activity can lead to degenerative pathologies and cancers. Our understanding of calpain function is based primarily on typical calpains, which carry EF hand motifs that bind Ca²⁺ or mediate dimerization; however, typical and atypical calpains, which lack EF hand motifs, are both present in mammals. Hence, any therapeutic intervention designed to suppress degenerative conditions, particularly those caused by elevated Ca²⁺ levels, should also consider the potential involvement of atypical calpains. We have taken advantage of the model organism C. elegans, which only encodes atypical calpain proteins, to gain an understanding of the evolution and activities of these proteins. We show that the CLP-1 atypical calpain is normally expressed in muscle and localizes to sarcomeric sub-structures. We find that CLP-1 contributes to the muscle degeneration observed in a model of Duchenne muscular dystrophy. Our studies also highlight the importance of calcium dysregulation in promoting CLP-1 activity and muscle degeneration.

Eukaryotic mechanosensitive channels

Mechanosensitive ion channels are gated directly by physical stimuli and transduce these stimuli into electrical signals. Several criteria must apply for a channel to be considered mechanically gated. Mechanosensitive channels from bacterial systems have met these criteria, but few eukaryotic channels have been confirmed by the same standards. Recent work has suggested or confirmed that diverse types of channels, including TRP channels, K(2P) channels, MscS-like proteins, and DEG/ENaC channels, are mechanically gated. Several studies point to the importance of the plasma membrane for channel gating, but intracellular and/or extracellular structures may also be required.

Stomatin-like protein-1 interacts with stomatin and is targeted to late endosomes

The human stomatin-like protein-1 (SLP-1) is a membrane protein with a characteristic bipartite structure containing a stomatin domain and a sterol carrier protein-2 (SCP-2) domain. This structure suggests a role for SLP-1 in sterol/lipid transfer and transport. Because SLP-1 has not been investigated, we first studied the molecular and cell biological characteristics of the expressed protein. We show here that SLP-1 localizes to the late endosomal compartment, like stomatin. Unlike stomatin, SLP-1 does not localize to the plasma membrane. Overexpression of SLP-1 leads to the redistribution of stomatin from the plasma membrane to late endosomes suggesting a complex formation between these proteins. We found that the targeting of SLP-1 to late endosomes is caused by a GYXXPhi (Phi being a bulky, hydrophobic amino acid) sorting signal at the N terminus. Mutation of this signal results in plasma membrane localization. SLP-1 and stomatin co-localize in the late endosomal compartment, they co-immunoprecipitate, thus showing a direct interaction, and they associate with detergent-resistant membranes. In accordance with the proposed lipid transfer function, we show that, under conditions of blocked cholesterol efflux from late endosomes, SLP-1 induces the formation of enlarged, cholesterol-filled, weakly LAMP-2-positive, acidic vesicles in the perinuclear region. This massive cholesterol accumulation clearly depends on the SCP-2 domain of SLP-1, suggesting a role for this domain in cholesterol transfer to late endosomes.

Gene expression profiling: Canonical molecular changes and clinicopathological features in sporadic colorectal cancers

AIM: To investigate alternative or subordinate pathways involved in colorectal tumorigenesis and tumor growth, possibly determining at-risk populations and predicting responses to treatment.

METHODS: Using microarray gene-expression analysis, we analyzed patterns of gene expression relative to canonical molecular changes and clinicopathological features in 84 sporadic colorectal cancer patients, standardized by tumor location. Subsets of differentially expressed genes were confirmed by real-time reverse-transcript polymerase chain reaction (RT-PCR).

RESULTS: The largest number of genes identified as being differentially expressed was by tumor location, and the next largest number by lymphovascular or neural invasion of tumor cells and by mismatch repair (MMR) defects. Amongst biological processes, the immune response was significantly implicated in entire molecular changes observed during colorectal tumorigenesis (P < 0.001). Amongst 47 differentially expressed genes, seven (PISD, NIBP, BAI2, STOML1, MRPL21, MRPL16, and MKKS) were newly found to correlate with tumorigenesis and tumor growth. Most location-associated molecular changes had distinct effects on gene expression, but the effects of the latter were sometimes contradictory.

CONCLUSION: We show that several differentially expressed genes were associated with canonical molecular changes in sporadic colorectal cancers, possibly constituting alternative or subordinate pathways of tumorigenesis. As tumor location was the dominant factor influencing differential gene expression, location-specific analysis may identify location-associated pathways and enhance the accuracy of class prediction.

Nanoscale organization of the MEC-4 DEG/ENaC sensory mechanotransduction channel in Caenorhabditis elegans touch receptor neurons

Hearing, touch and proprioception are thought to involve direct activation of mechano-electrical transduction (MeT) channels. In Caenorhabditis elegans touch receptor neurons (TRNs), such channels contain two pore-forming subunits (MEC-4 and MEC-10) and two auxiliary subunits (MEC-2 and MEC-6). MEC-4 and MEC-10 belong to a large superfamily of ion channel proteins (DEG/ENaCs) that form nonvoltage-gated, amiloride-sensitive Na+ channels. In TRNs, unique 15-protofilament microtubules and an electron-dense extracellular matrix have been proposed to serve as gating tethers critical for MeT channel activation. We combined high-pressure freezing and serial-section immunoelectron microscopy to determine the position of MeT channels relative to putative gating tethers. MeT channels were visualized using antibodies against MEC-4 and MEC-2. This nanometer-resolution view of a sensory MeT channel establishes structural constraints on the mechanics of channel gating. We show here that MEC-2 and MEC-5 collagen, a putative extracellular tether, occupy overlapping but distinct domains in TRN neurites. Although channels decorate all sides of TRN neurites; they are not associated with the distal endpoints of 15-protofilament microtubules hypothesized to be gating tethers. These specialized microtubules, which are unique to TRNs, assemble into a cross-linked bundle connected by a network of kinked filaments to the neurite membrane. We speculate that the microtubule bundle converts external point loads into membrane stretch which, in turn, facilitates MeT channel activation.

Touch sensitivity in Caenorhabditis elegans

The nematode Caenorhabditis elegans was the first organism for which touch insensitive mutants were obtained. The study of the genes defective in these mutants has led to the identification of components of a mechanosensory complex needed for specific cells to sense gentle touch to the body. Multiple approaches using genetics, cell biology, biochemistry, and electrophysiology have characterized a channel complex, containing two DEG/ENaC pore-forming subunits and several other proteins, that transduces the touch response. Other mechanical responses, sensed by other cells using a variety of other components, are less well understood in C. elegans. Many of these other senses may use TRP channels, although DEG/ENaC channels have also been implicated.

Mechanosensitive Ion Channels in Caenorhabditis elegans

Caenorhabditis elegans depends critically on mechanosensory perception to negotiate its natural habitat, the soil. The worm displays a rich repertoire of mechanosensitive behaviors, which can be easily examined in the laboratory. This, coupled with the availability of sophisticated genetic and molecular biology tools, renders C. elegans a particularly attractive model organism to study the transduction of mechanical stimuli to biological responses. Systematic genetic analysis has facilitated the dissection of the molecular mechanisms that underlie mechanosensation in the nematode. Studies of various worm mechanosensitive behaviors have converged to identify highly specialized plasma membrane ion channels that are required for the conversion of mechanical energy to cellular signals. Strikingly, similar mechanosensitive ion channels appear to function at the core of the mechanotransduction apparatus in higher organisms, including humans. Thus, the mechanisms responsible for the detection of mechanical stimuli are likely conserved across metazoans. The nematode offers a powerful platform for elucidating the fundamental principles that govern the function of metazoan mechanotransducers. This chapter evaluates the current understanding of mechanotransduction in C. elegans and focuses on the role of mechanosensitive ion channels in specific mechanosensory behavioral responses. The chapter also outlines potential unifying themes, common to mechanosensory transduction in diverse species.

Podocin and MEC-2 bind cholesterol to regulate the activity of associated ion channels

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: DEG/ENaC 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.

Sulfated signal from ASJ sensory neurons modulates stomatin-dependent coordination in Caenorhabditis elegans

The neuronal stomatin-like proteins UNC-1 and UNC-24 play important roles in the nervous system of Caenorhabditis elegans. These neuronal stomatin-like proteins are putative chaperone proteins that can modify volatile anesthetic sensitivity and disrupt coordinated locomotion. A suppressor of unc-1 and unc-24, named ssu-1(fc73) (for suppressor of stomatin uncoordination), suppresses three phenotypes of neuronal stomatin-like protein deficiency as follows: volatile anesthetic sensitivity, uncoordinated locomotion, and a constitutive alternative developmental phenotype known as dauer. Here we provide the first phenotypic characterization of ssu-1, predicted to be the only C. elegans cytosolic alcohol sulfotransferase, a family of enzymes that catalyze a sulfate linkage with the alcohol group of small molecules for the purposes of detoxification or modification of signaling. In vitro enzyme analysis of bacterially expressed SSU-1 demonstrates sulfotransferase activity and thus confirms the function predicted by protein sequence similarities. Whereas unc-1 is expressed in the majority of neurons of C. elegans, expression of SSU-1 protein in only the two ASJ amphid interneurons is sufficient to restore the wild type phenotype. This work demonstrates that SSU-1 is a functional sulfotransferase that likely modifies endocrine signaling in C. elegans. The expression of SSU-1 in the ASJ neurons refines the understanding of the function of these cells and supports their classification as endocrine tissue. The relationship of unc-1, unc-24, and ssu-1 is the first association of neuronal stomatin-like proteins sharing regulatory roles with a sulfotransferase enzyme.

Phospholipid transfer proteins in perspective

Since their discovery and subsequent purification from mammalian tissues more than 30 years ago an impressive number of studies have been carried out to characterize and elucidate the biological functions of phosphatidylcholine transfer protein (PC-TP), phosphatidylinositol transfer protein (PI-TP) and non-specific lipid transfer protein, more commonly known as sterol carrier protein 2 (SCP-2). Here I will present information to show that these soluble, low-molecular weight proteins constitute domain structures in StArR-related lipid transfer (START) proteins (i.e. PC-TP), in retinal degeneration protein, type B (RdgB)-related PI-TPs (e.g. Dm RdgB, Nir2, Nir3) and in peroxisomal beta-oxidation enzyme-related SCP-2 (i.e. 3-oxoacyl-CoA thiolase, also denoted as SCP-X and the 80-kDa D-bifunctional protein). Further I will summarize the most recent studies pertaining to the physiological function of these soluble phospholipid transfer proteins in metazoa.

Fusion and fission, the evolution of sterol carrier protein-2

Sterol carrier protein-2 (SCP-2) is an intracellular, small, basic protein domain that in vitro enhances the transfer of lipids between membranes. It is expressed in bacteria, archaea, and eukaryotes. There are five human genes, HSD17B4, SCPX, HSDL2 STOML1, and C20orf79, which encode SCP-2. HSD17B4, SCPX, HSDL2, and STOML1 encode fusion proteins with SCP-2 downstream of another protein domain, whereas C20orf79 encodes an unfused SCP-2. We have extracted SCP-2 domains from databases and analyzed the evolution of the eukaryotic SCP-2. We show that SCPX and HSDL2 are present in most animals from Cnidaria to Chordata. STOML1 are present in nematodes and more advanced animals. HSD17B4 which encodes a D-bifunctional protein (DBP) with domains for D-3-hydroxyacyl-CoA dehydrogenase, enoyl-CoA hydratase, and SCP-2 are found in animals from insects to mammals and also in fungi. Nematodes, amoebas, ciliates, apicomplexans, and oomycetes express an alternative DBP with the SCP-2 domain directly connected to the D-3-hydroxyacyl-CoA dehydrogenase. This fusion has not been retained in plant genomes, which solely express unfused SCP-2 domains. Proteins carrying unfused SCP-2 domains are also encoded in bacteria, archaea, ciliates, fungi, insects, nematodes, and vertebrates. Our results indicate that the fusion between D-3-hydroxyacyl-CoA dehydrogenase and SCP-2 was formed early during eukaryotic evolution. There have since been several gene fission events where genes encoding unfused SCP-2 domains have been formed, as well as gene fusion events placing the SCP-2 domain in novel protein domain contexts.

Flotillins and the PHB Domain Protein Family: Rafts, Worms and Anaesthetics

While our understanding of lipid microdomains has advanced in recent years, many aspects of their formation and dynamics are still unclear. In particular, the molecular determinants that facilitate the partitioning of integral membrane proteins into lipid raft domains are yet to be clarified. This review focuses on a family of raft-associated integral membrane proteins, termed flotillins, which belongs to a larger class of integral membrane proteins that carry an evolutionarily conserved domain called the prohibitin homology (PHB) domain. A number of studies now suggest that eucaryotic proteins carrying this domain have affinity for lipid raft domains. The PHB domain is carried by a diverse array of proteins including stomatin, podocin, the archetypal PHB protein, prohibitin, lower eucaryotic proteins such as the Dictyostelium discoideum proteins vacuolin A and vacuolin B and the Caenorhabditis elegans proteins unc-1, unc-24 and mec-2. The presence of this domain in some procaryotic proteins suggests that the PHB domain may constitute a primordial lipid recognition motif. Recent work has provided new insights into the trafficking and targeting of flotillin and other PHB domain proteins. While the function of this large family of proteins remains unclear, studies of the C. elegans PHB proteins suggest possible links to a class of volatile anaesthetics raising the possibility that these lipophilic agents could influence lipid raft domains. This review will discuss recent insights into the cell biology of flotillins and the large family of evolutionarily conserved PHB domain proteins.

Plants Express a Lipid Transfer Protein with High Similarity to Mammalian Sterol Carrier Protein-2

This is the first report describing the cloning and characterization of sterol carrier protein-2 (SCP-2) from plants. Arabidopsis thaliana SCP-2 (AtSCP-2) consists of 123 amino acids with a molecular mass of 13.6 kDa. AtSCP-2 shows 35% identity and 56% similarity to the human SCP-2-like domain present in the human D-bifunctional protein (DBP) and 30% identity and 54% similarity to the human SCP-2 encoded by SCP-X. The presented structural models of apo-AtSCP-2 and the ligand-bound conformation of AtSCP-2 reveal remarkable similarity with two of the structurally known SCP-2s, the SCP-2-like domain of human DBP and the rabbit SCP-2, correspondingly. The AtSCP-2 models in both forms have a similar hydrophobic ligand-binding tunnel, which is extremely suitable for lipid binding. AtSCP-2 showed in vitro transfer activity of BODIPY-phosphatidylcholine (BODIPY-PC) from donor membranes to acceptor membranes. The transfer of BODIPY-PC was almost completely inhibited after addition of 1-palmitoyl 2-oleoyl phosphatidylcholine or ergosterol. Dimyristoyl phosphatidic acid, stigmasterol, steryl glucoside, and cholesterol showed a moderate to marginal ability to lower the BODIPY-PC transfer rate, and the single chain palmitic acid and stearoyl-coenzyme A did not affect transfer at all. Expression analysis showed that AtSCP-2 mRNA is accumulating in most plant tissues. Plasmids carrying fusion genes between green fluorescent protein and AtSCP-2 were transformed with particle bombardment to onion epidermal cells. The results from analyzing the transformants indicate that AtSCP-2 is localized to peroxisomes.

MEC-2 Is Recruited to the Putative Mechanosensory Complex in C. elegans Touch Receptor Neurons through Its Stomatin-like Domain

BACKGROUND

The response to gentle body touch in C. elegans requires a degenerin channel complex containing four proteins (MEC-2, MEC-4, MEC-6, and MEC-10). The central portion of the integral membrane protein MEC-2 contains a stomatin-like region that is highly conserved from bacteria to mammals. The molecular function of this domain in MEC-2, however, is unknown.

RESULTS

Here, we show that MEC-2 colocalizes with the degenerin MEC-4 in regular puncta along touch receptor neuron processes. This punctate localization requires the other channel complex proteins. The stomatin-like region of MEC-2 interacts with the intracellular cytoplasmic portion of MEC-4. Missense mutations in this region that destroy the interaction also disrupt the punctate localization and degenerin-regulating function of MEC-2. Missense mutations outside this region apparently have no effect on the punctate localization but significantly reduce the regulatory effect of MEC-2 on the MEC-4 degenerin channel. A second stomatin-like protein, UNC-24, colocalizes with MEC-2 in vivo and coimmunoprecipitates with MEC-2 and MEC-4 in Xenopus oocytes; unc-24 enhances the touch insensitivity of temperature-sensitive alleles of mec-4 and mec-6.

CONCLUSION

Two stomatin homologs, MEC-2 and UNC-24, interact with the MEC-4 degenerin through their stomatin-like regions, which act as protein binding domains. At least in the case of MEC-2, this binding allows its nonstomatin domains to regulate channel activity. Stomatin-like regions in other proteins may serve a similar protein binding function.

Expression of a sterol carrier protein-x gene in the yellow fever mosquito, Aedes aegypti

The sterol carrier protein-x (SCP-x), a peroxisomal thiolase/nonspecific lipid binding protein, was characterized in the yellow fever mosquito, Aedes aegypti. The Aedes aegypti SCP-x (AeSCP-x) has 83% and 75% similarities to Drosophila and mammalian SCP-x, respectively. However, the AeSCP-x gene did not produce multiple transcripts, which is characteristic of the vertebrate SCP-x gene. Levels of AeSCP-x transcription were higher in larvae and pupae. Gut tissue showed the highest level of AeSCP-x mRNA in larvae. In adults, low levels of AeSCP-x transcription were detected in both sexes. Polyclonal antibodies against the sterol carrier protein-2 (SCP-2) domain of AeSCP-x detected two proteins of 62 kDa and 13 kDa. The results indicate that AeSCP-x is proteolytically cleaved after translation to produce a smaller protein that contains only the SCP-2 domain, which is similar to post-translational modification of the vertebrate's SCP-x to produce multiple products.

Genetic Models of Mechanotransduction: The NematodeCaenorhabditis elegans

Mechanotransduction, the conversion of a mechanical stimulus into a biological response, constitutes the basis for a plethora of fundamental biological processes such as the senses of touch, balance, and hearing and contributes critically to development and homeostasis in all organisms. Despite this profound importance in biology, we know remarkably little about how mechanical input forces delivered to a cell are interpreted to an extensive repertoire of output physiological responses. Recent, elegant genetic and electrophysiological studies have shown that specialized macromolecular complexes, encompassing mechanically gated ion channels, play a central role in the transformation of mechanical forces into a cellular signal, which takes place in mechanosensory organs of diverse organisms. These complexes are highly efficient sensors, closely entangled with their surrounding environment. Such association appears essential for proper channel gating and provides proximity of the mechanosensory apparatus to the source of triggering mechanical energy. Genetic and molecular evidence collected in model organisms such as the nematode worm Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the mouse highlight two distinct classes of mechanically gated ion channels: the degenerin (DEG)/epithelial Na+channel (ENaC) family and the transient receptor potential (TRP) family of ion channels. In addition to the core channel proteins, several other potentially interacting molecules have in some cases been identified, which are likely parts of the mechanotransducing apparatus. Based on cumulative data, a model of the sensory mechanotransducer has emerged that encompasses our current understanding of the process and fulfills the structural requirements dictated by its dedicated function. It remains to be seen how general this model is and whether it will withstand the impiteous test of time.

Eukaryotic and prokaryotic stomatins: the proteolytic link

The 32kD membrane protein stomatin was first studied because it is deficient from the red cell membrane in two forms of the class of haemolytic anaemias known as "hereditary stomatocytosis." The hallmark of these conditions is a plasma membrane leak to the monovalent cations Na+ and K+: the protein is missing only in the most severely leaky of these conditions. No mutation has ever been found in the stomatin gene in these conditions. Stomatin-like proteins have been identified in all three domains of biology, yet their function remains enigmatic. Although the murine knock-out is without phenotype, we have identified a family showing a splicing defect in the stomatin mRNA, in which affected children showed a catastrophic multisystem disease not inconsistent with the now-known wide tissue distribution of stomatin. We report here a study of strongly homologous stomatin-like genes in prokaryotes, which reveals a close connection with a never-studied gene erroneously known as "nfed." This gene codes for a hydrophobic protein with a probable serine protease motif. It is possible that these stomatin-like genes and those which are known as"nfed" form an operon, suggesting that the two protein products are aimed at a common function. The corollary is that stomatin could be a partner protein for a membrane-bound proteolytic process, in both prokaryotes and in eukaryotes generally: this idea is consistent with experimental evidence.

A stomatin and a degenerin interact in lipid rafts of the nervous system of Caenorhabditis elegans

In Caenorhabditis elegans, the gene unc-1 controls anesthetic sensitivity and normal locomotion. The protein UNC-1 is a close homolog of the mammalian protein stomatin and is expressed primarily in the nervous system. Genetic studies in C. elegans have shown that the UNC-1 protein interacts with a sodium channel subunit, UNC-8. In humans, absence of stomatin is associated with abnormal sodium and potassium levels in red blood cells. Stomatin also has been postulated to participate in the formation of lipid rafts, which are membrane microdomains associated with protein complexes, cholesterol, and sphingolipids. In this study, we isolated a low-density, detergent-resistant fraction from cell membranes of C. elegans. This fraction contains cholesterol, sphingolipids, and protein consistent with their identification as lipid rafts. We then probed Western blots of protein from the rafts and found that the UNC-1 protein is almost totally restricted to this fraction. The UNC-8 protein is also found in rafts and coimmunoprecipitates UNC-1. A second stomatin-like protein, UNC-24, also affects anesthetic sensitivity, is found in lipid rafts, and regulates UNC-1 distribution. Mutations in the unc-24 gene alter the distribution of UNC-1 in lipid rafts. Each of these mutations alters anesthetic sensitivity in C. elegans. Because lipid rafts contain many of the putative targets of volatile anesthetics, they may represent a novel class of targets for volatile anesthetics.

Identification of a stomatin orthologue in vacuoles induced in human erythrocytes by malaria parasites. A role for microbial raft proteins in apicomplexan vacuole biogenesis

When the human malaria parasite Plasmodium falciparum infects erythrocytes, proteins associated with host-derived detergent-resistant membrane (DRM) rafts are selectively recruited into the newly formed vacuole, but parasite proteins that contribute to raft-based vacuole development are unknown. In mammalian cells, DRM-associated integral membrane proteins such as caveolin-1 and flotillin-1 that form oligomers have been linked to the formation of DRM-based invaginations called caveolae. Here we show that the P. falciparum genome does not encode caveolins or flotillins but does contain an orthologue of human band 7 stomatin, a protein known to oligomerize, associate with non-caveolar DRMs and is distantly related to flotillins. Stomatins are members of a large protein family conserved in evolution and P. falciparum (Pf) stomatin appears to be a prokaryotic-like molecule. Evidence is presented that it associates with DRMs and may oligomerize, suggesting that these features are conserved in the stomatin family. Further, Pfstomatin is an integral membrane protein concentrated at the apical end of extracellular parasites, where it co-localizes with invasion-associated rhoptry organelles. A resident rhoptry protein, RhopH2 also resides in DRMs. This provides the first evidence that rhoptries of an apicomplexan parasite contain DRM rafts. Further, when the parasite invades erythrocytes, rhoptry Pfstomatin and RhopH2 are inserted into the newly formed vacuole. Thus, like caveolin-1 and flotillin-1, a stomatin may also associate with non-clathrin coated, DRM-enriched vacuoles. We propose a new model of invasion and vacuole formation involving DRM-based interactions of both host and parasite molecules.

Proteomic analysis of the mouse liver mitochondrial inner membrane

Mitochondria play a crucial role in cellular homeostasis, which justifies the increasing interest in mapping the different components of these organelles. Here we have focused our study on the identification of proteins of the mitochondrial inner membrane (MIM). This membrane is of particular interest because, besides the well known components of the respiratory chain complexes, it contains several ion channels and many carrier proteins that certainly play a key role in mitochondrial function and, therefore, deserve to be identified at the molecular level. To achieve this goal we have used a novel approach combining the use of highly purified mouse liver mitochondrial inner membranes, extraction of membrane proteins with organic acid, and two-dimensional liquid chromatography coupled to tandem mass spectrometry. This procedure allowed us to identify 182 proteins that are involved in several biochemical processes, such as the electron transport machinery, the protein import machinery, protein synthesis, lipid metabolism, and ion or substrate transport. The full range of isoelectric point (3.9-12.5), molecular mass (6-527 kDa), and hydrophobicity values (up to 16 transmembrane predicted domains) were represented. In addition, of the 182 proteins found, 20 were unknown or had never previously been associated with the MIM. Overexpression of some of these proteins in mammalian cells confirmed their mitochondrial localization and resulted in severe remodeling of the mitochondrial network. This study provides the first proteome of the MIM and provides a basis for a more detailed study of the newly characterized proteins of this membrane.

Identification of genes expressed in C. elegans touch receptor neurons

The extent of gene regulation in cell differentiation is poorly understood. We previously used saturation mutagenesis to identify 18 genes that are needed for the development and function of a single type of sensory neuron--the touch receptor neuron for gentle touch in Caenorhabditis elegans. One of these genes, mec-3, encodes a transcription factor that controls touch receptor differentiation. By culturing and isolating wild-type and mec-3 mutant cells from embryos and applying their amplified RNA to DNA microarrays, here we have identified genes that are known to be expressed in touch receptors, a previously uncloned gene (mec-17) that is needed for maintaining touch receptor differentiation, and more than 50 previously unknown mec-3-dependent genes. These genes are randomly distributed in the genome and under-represented both for genes that are co-expressed in operons and for multiple members of gene families. Using regions 5' of the start codon of the first 20 genes, we have also identified an over-represented heptanucleotide, AATGCAT, that is needed for the expression of touch receptor genes.

Crystal structure of the liganded SCP-2-like domain of human peroxisomal multifunctional enzyme type 2 at 1.75 A resolution

beta-Oxidation of amino acyl coenzyme A (acyl-CoA) species in mammalian peroxisomes can occur via either multifunctional enzyme type 1 (MFE-1) or type 2 (MFE-2), both of which catalyze the hydration of trans-2-enoyl-CoA and the dehydrogenation of 3-hydroxyacyl-CoA, but with opposite chiral specificity. MFE-2 has a modular organization of three domains. The function of the C-terminal domain of the mammalian MFE-2, which shows similarity with sterol carrier protein type 2 (SCP-2), is unclear. Here, the structure of the SCP-2-like domain comprising amino acid residues 618-736 of human MFE-2 (d Delta h Delta SCP-2L) was solved at 1.75 A resolution in complex with Triton X-100, an analog of a lipid molecule. This is the first reported structure of an MFE-2 domain. The d Delta h Delta SCP-2L has an alpha/beta-fold consisting of five beta-strands and five alpha-helices; the overall architecture resembles the rabbit and human SCP-2 structures. However, the structure of d Delta h Delta SCP-2L shows a hydrophobic tunnel that traverses the protein, which is occupied by an ordered Triton X-100 molecule. The tunnel is large enough to accommodate molecules such as straight-chain and branched-chain fatty acyl-CoAs and bile acid intermediates. Large empty apolar cavities are observed near the exit of the tunnel and between the helices C and D. In addition, the C-terminal peroxisomal targeting signal is ordered in the structure and solvent-exposed, which is not the case with unliganded rabbit SCP-2, supporting the hypothesis of a ligand-assisted targeting mechanism.

Gene structure, intracellular localization, and functional roles of sterol carrier protein-2

Since its discovery three decades ago, sterol carrier protein-2 (SCP-2) has remained a fascinating protein whose physiological function in lipid metabolism remains an enigma. Its multiple proposed functions arise from its complex gene structure, post-translational processing, intracellular localization, and ligand specificity. The SCP-2 gene has two initiation sites coding for proteins that share a common 13 kDa SCP-2 C-terminus: (1) One site codes for 58 kDa SCP-x which is partially post-translationally cleaved to 13 kDa SCP-2 and a 45 kDa protein. (2) A second site codes for 15 kDa pro-SCP-2 which is completely post-translationally cleaved to 13 kDa SCP-2. Very little is yet known regarding how the relative proportions of the two transcripts are regulated. Although all three proteins contain a C-terminal SKL peroxisomal targeting sequence, it is unclear why all three proteins are not exclusively localized in peroxisomes. However, the recent demonstration that the SCP-2 N-terminal presequence in pro-SCP-2 dramatically modulated the intracellular targeting coded by the C-terminal peroxisomal targeting sequence may account for the observation that as much as half of total SCP-2 is localized outside the peroxisome. The tertiary and secondary structure of the 13 kDa SCP-2, but not that of 15 kDa pro-SCP-2 and 58 kDa SCP-x, are now resolved. Increasing evidence suggests that the 58 kDa SCP-x and 45 kDa proteins are peroxisomal 3-ketoacyl-CoA-thiolases involved in the oxidation of branched chain fatty acids. Since 15 kDa pro-SCP-2 is post-translationally completely cleaved to 13 kDa SCP-2, relatively little attention has been focused on this protein. Finally, although the 13 kDa SCP-2 is the most studied of these proteins, because it exhibits diversity of its ligand partners (fatty acids, fatty acyl CoAs, cholesterol, phospholipids), new potential physiological function(s) are still being proposed and questions regarding potential compensation by other proteins with overlapping specificity are only beginning to be resolved.

Model organisms: new insights into ion channel and transporter function. Stomatin homologues interact in Caenorhabditis elegans

In C. elegans the protein UNC-1 is a major determinant of anesthetic sensitivity and is a close homologue of the mammalian protein stomatin. In humans stomatin is missing from erythrocyte membranes in the hemolytic disease overhydrated hereditary stomatocytosis, despite an apparently normal stomatin gene. Overhydrated hereditary stomatocytosis is characterized by alteration of the normal transmembrane gradients of sodium and potassium. Stomatin has been shown to interact genetically with sodium channels. It is also postulated that stomatin is important in the organization of lipid rafts. We demonstrate here that antibodies against UNC-1 stain the major nerve tracts of Caenorhabditis elegans, with very intense staining of the nerve ring. We also found that a gene encoding a stomatin-like protein, UNC-24, affects anesthetic sensitivity and is genetically epistatic to unc-1. In the absence of UNC-24, the staining of the nerve ring by anti-UNC-1 is abolished, despite normal transcriptional levels of the unc-1 mRNA. Western blots indicate that UNC-24 probably affects the stability of the UNC-1 protein. UNC-24 may therefore be necessary for the correct placement of UNC-1 in the cell membrane and organization of lipid rafts.

Analysis of a 1-megabase deletion in 15q22-q23 in an autistic patient: identification of candidate genes for autism and of homologous DNA segments in 15q22-q23 and 15q11-q13

We have identified a one megabase deletion in the 15q22-15q23 region in a patient with autism, developmental delay, and mild dysmorphism. Genes that map within the deletion region and genes that are interrupted or rearranged at the deletion breakpoints are candidate genes for autism. Fluroescence in situ hybridization studies in this patient revealed that part or all of the PML gene is absent from one chromosome 15 and a BAC clone containing the D15S124 gene locus hybridizes to only one chromosome 15. BAC clones containing the PTPN9, and SLP-1[hUNC24] genes showed markedly reduced hybridization in the 15q22-q23 region on one chromosome 15 in the patient. These BACs also hybridize to the 15q11-q13 region in close proximity to SNRPN and HERC2, and in this region there is equal intensity of signal on the normal and on the deleted chromosome. There are previous reports of deletions and duplications of the 15q11-q13 region in patients with autism. Our patient represents the first report of a 15q22-q23 deletion. Hybridization of the PTPN9 and Slp-1 Bac clones to the 15q11-q13 and the 15q22-q23 regions of chromosome 15 may be due to the presence of PTPN9 or SLP-1 gene sequences or to the presence of other gene sequences or to non-coding homologous DNA sequences. The PTPN9 gene encodes a non-receptor protein tyrosine phosphatase. The Slp-1 [hUNC24] gene is expressed mainly in the brain. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:765-770, 2000.

Sterol carrier protein-2

The compartmentalization of cholesterol metabolism implies target-specific cholesterol trafficking between the endoplasmic reticulum, plasma membrane, lysosomes, mitochondria and peroxisomes. One hypothesis has been that sterol carrier protein-2 (SCP2, also known as the non-specific lipid transfer protein) acts in cholesterol transport through the cytoplasm. Recent studies employing gene targeting in mice showed, however, that mice lacking SCP2 and the related putative sterol carrier known as SCPx, develop a defect in peroxisomal beta-oxidation. In addition, diminished peroxisomal alpha-oxidation of phytanic acid (3,7,11, 15-tetramethylhexadecanoic acid) in these null mice was attributed to the absence of SCP2 which has a number of properties supporting a function as carrier for fatty acyl-CoAs rather than for sterols.

Identification and characterization of human SLP-2, a novel homologue of stomatin (band 7.2b) present in erythrocytes and other tissues

Human stomatin (band 7.2b) is a 31-kDa erythrocyte membrane protein of unknown function but implicated in the control of ion channel permeability, mechanoreception, and lipid domain organization. Although absent in erythrocytes from patients with hereditary stomatocytosis, stomatin is not linked to this disorder. A second stomatin homologue, termed SLP-1, has been identified in nonerythroid tissues, and other stomatin related proteins are found in Drosophila, Caenorhabditis elegans, and plants. We now report the cloning and characterization of a new and unusual stomatin homologue, human SLP-2 (stomatin-like protein 2). SLP-2 is encoded by an approximately 1.5-kilobase mRNA (GenBank(TM) accession no. AF190167). The gene for human SLP-2, HUSLP2, is present on chromosome 9p13. Its derived amino acid sequence predicts a 38,537-kDa protein that is overall approximately 20% similar to human stomatin. Northern and Western blots for SLP-1 and SLP-2 reveal a wide but incompletely overlapping tissue distribution. Unlike SLP-1, SLP-2 is also present in mature human erythrocytes ( approximately 4,000 +/- 5,600 (+/- 2 S.D.) copies/cell). SLP-2 lacks a characteristic NH(2)-terminal hydrophobic domain found in other stomatin homologues and (unlike stomatin) is fully extractable from erythrocyte membranes by NaOH, pH 11. SLP-2 partitions into both Triton X-100-soluble and -insoluble pools in erythrocyte ghost membranes or when expressed in cultured COS cells and migrates anomalously on SDS-polyacrylamide gel electrophoresis analysis with apparent mobilities of approximately 45,500, 44,600, and 34,300 M(r). The smallest of these protein bands is believed to represent the product of alternative translation initiated at AUGs beginning with nt 217 or 391, although this point has not been rigorously proven. Collectively, these findings identify a novel and unusual member of the stomatin gene superfamily that interacts with the peripheral erythrocyte cytoskeleton and presumably other integral membrane proteins but not directly with the membrane bilayer. We hypothesize that SLP-2 may link stomatin or other integral membrane proteins to the peripheral cytoskeleton and thereby play a role in regulating ion channel conductances or the organization of sphingolipid and cholesterol-rich lipid rafts.

NMR structure of the sterol carrier protein-2: implications for the biological role

The determination of the NMR structure of the sterol carrier protein-2 (SCP2), analysis of backbone (15)N spin relaxation parameters and NMR studies of nitroxide spin-labeled substrate binding are presented as a new basis for investigations of the mode of action of SCP2. The SCP2 fold is formed by a five-stranded beta-sheet and four alpha-helices. Fatty acid binding to a hydrophobic surface area formed by amino acid residues of the first and third helices, and the beta-sheet, which are all located in the polypeptide segment 8-102, was identified with the use of the spin-labeled substrate 16-doxylstearic acid. In the free protein, the lipid-binding site is covered by the C-terminal segment 105-123, suggesting that this polypeptide segment, which carries the peroxisomal targeting signal (PTS1), might be involved in the regulation of ligand binding.

Association of stomatin with lipid-protein complexes in the plasma membrane and the endocytic compartment

Membrane protein - microvilli - lipid raft - GPI-anchored protein - epithelial cell The 31 kDa integral membrane protein stomatin (protein 7.2b) has a monotopic structure and a cytofacial orientation. We have shown previously that stomatin is located in plasma membrane protruding structures and forms high-order homo-oligomers in the human epithelial cell line UAC, suggesting that this protein has a structural function in the cortical morphogenesis of the cells. It is also present in a pool of juxtanuclear vesicles. In this study, we show that stomatin colocalizes with the GPI-anchored proteins placental alkaline phosphatase (PLAP) and membrane folate receptor alpha (MFRalpha) endogenously expressed in UAC cells. This observation enabled us to demonstrate two different aspects of stomatin. First, using anti-PLAP antibody internalization, we show that the peri-centrosomal vesicles containing stomatin correspond to a subset of endosomes, which can also be labeled with the late endosomal/lysosomal marker LAMP-2. Secondly, we found that stomatin is partially present in detergent-insoluble membrane domains and co-patches with PLAP on the plasma membrane, after cross-linking of PLAP by antibodies. These data indicate that stomatin and GPI-anchored proteins are linked through lipid rafts and undergo the same sorting events. We propose that stomatin, through its affinity for lipid rafts, functions in concentrating GPI-anchored proteins in membrane microvillar structures. Consistent with this hypothesis, we found that stomatin is expressed exclusively in microvilli of the apical membrane in polarized Madin-Darby canine kidney (MDCK) cells.

Temperature effects on cation transport in hereditary stomatocytosis and allied disorders

The conditions known as 'hereditary stomatocytosis and allied syndromes' comprise a group of dominantly inherited human haemolytic anaemias characterized by a plasma membrane 'leak' to the univalent cations Na and K, an example of a small but growing group of diseases where pathology can be directly attributed to abnormal membrane transport. A number of case reports in the different variants have alluded to temperature-related phenomena, including loss of K on storage at room temperature (giving 'pseudohyperkalaemia') and lysis of cells when stored in the cold ('cryohydrocytosis'). This review collects together published studies of these temperature effects, which show very major differences in the 'leak' K transport. Two main variations on normal emerge: a 'shallow slope' type, in which the flux shows an abnormally low dependence on temperature in the range 37-20 degrees C, and 'high minimum', in which the minimum in this flux, which occurs in normal cells at 8 degrees C, is shifted up to 23 degrees C. These temperature studies provide a powerful method for phenotypic characterization.

Stomatin, a MEC-2 like protein, is expressed by mammalian sensory neurons

The molecular mechanism whereby vertebrate primary sensory neurons convert mechanical energy at their receptive fields into action potentials is unknown. In recent years, genetic screens for touch insensitive mutants of the nematode worm Caenorhabditis elegans have led to the identification of several genes required for mechanical sensitivity. A model has been proposed in which a mechanically gated ion channel is connected both to the extracellular matrix and to the cytoskeleton. Displacement of the membrane is proposed to produce a shearing force that pulls the channel open. MEC-2 is thought to play an important role in this complex by linking the ion channel to the cytoskeleton. MEC-2 is highly homologous to a vertebrate protein called stomatin. Stomatin was first isolated from erythrocytes where it is a major integral membrane protein. To date, however, no data on neuronal expression of stomatin in the peripheral nervous system (PNS) or central nervous system (CNS) is available. Here, we have used RT-PCR, in situ hybridization, Northern blotting, and immunocytochemistry to demonstrate that stomatin is expressed by all sensory neurons in mouse dorsal root ganglia. Indirect immunofluorescence together with transfection of cultured adult sensory neurons with epitope-tagged stomatin show that stomatin is localized in spots on somatic and axonal membranes. During development, stomatin begins to be expressed by sensory neurons only as target innervation occurs. The onset of expression of stomatin thus coincides with the onset of functional mechanical sensitivity. Together, our data suggest that stomatin, like the C. elegans MEC-2 gene, is expressed in an appropriate temporal and spatial manner to participate in a putative vertebrate mechanotransduction complex.

Stomatocytosis Is Absent in “Stomatin”-Deficient Murine Red Blood Cells

To examine the relationship between erythrocyte membrane protein 7.2b deficiency and the hemolytic anemia of human hereditary stomatocytosis, we created 7.2b knock-out mice by standard gene targeting approaches. Immunoblots showed that homozygous knock-out mice completely lacked erythrocyte protein 7.2b. Despite the absence of protein 7.2b, there was no hemolytic anemia and mouse red blood cells (RBCs) were normal in morphology, cell indices, hydration status, monovalent cation content, and ability to translocate lipids. The absence of the phenotype of hereditary stomatocytosis implies that protein 7.2b deficiency plays no direct role in the etiology of this disorder and casts doubt on the previously proposed role of this protein as a mediator of cation transport in RBC.

Stomatocytosis Is Absent in “Stomatin”-Deficient Murine Red Blood Cells

To examine the relationship between erythrocyte membrane protein 7.2b deficiency and the hemolytic anemia of human hereditary stomatocytosis, we created 7.2b knock-out mice by standard gene targeting approaches. Immunoblots showed that homozygous knock-out mice completely lacked erythrocyte protein 7.2b. Despite the absence of protein 7.2b, there was no hemolytic anemia and mouse red blood cells (RBCs) were normal in morphology, cell indices, hydration status, monovalent cation content, and ability to translocate lipids. The absence of the phenotype of hereditary stomatocytosis implies that protein 7.2b deficiency plays no direct role in the etiology of this disorder and casts doubt on the previously proposed role of this protein as a mediator of cation transport in RBC.

Hereditary dehydrated and overhydrated stomatocytosis: recent advances

The hereditary stomatocytoses and allied disorders are genetic defects of the erythrocyte membrane that result in abnormal permeability to the univalent cations Na+ and K+. Although rare, these conditions reflect abnormalities in physiologic mechanisms that are of paramount interest. All cases (as defined here) show increased plasma membrane permeability to Na+ and K+ and, to a greater or lesser degree, stomatocytic morphology. Dehydrated hereditary stomatocytosis, the most common form of hereditary stomatocytosis, is more heterogeneous than previously thought and includes kindreds showing pseudohyperkalemia or perinatal edema, or both. The gene responsible for both dehydrated hereditary stomatocytosis and familial pseudohyperkalemia, a nonhemolytic variant that presents with high plasma K+ levels, has been mapped to 16q23-qter. The cause of overhydrated hereditary stomatocytosis remains elusive despite the manifest lack of the enigmatic protein stomatin in the erythrocyte membrane. In all cases where splenectomy has been performed, this procedure has conferred a marked risk for thrombosis in adult life. This finding stresses the importance of diagnostic distinction between these conditions and hereditary spherocytosis.

Molecular cloning of hSLP-1, a novel human brain-specific member of the band 7/MEC-2 family similar to Caenorhabditis elegans UNC-24

We have isolated and characterized cDNA clones encoding a stomatin-like protein (hSLP-1) from a human cerebral cortex cDNA library. The deduced amino acid sequence (394 residues) revealed that hSLP-1 is a bipartite protein, containing a major stomatin-like part, starting at the N-terminus, and a non-specific lipid transfer protein (nsLTP)-domain at the C-terminal end, similar to the Caenorhabditis elegans protein UNC-24. Therefore, we conclude that hSLP-1 is the human homologue of UNC-24. In addition, the identification of an alternatively spliced variant demonstrated that two exon/intron boundaries are conserved in the hSLP-1 and unc-24 genes. Northern blot and RNA dot blot analyses showed that the 2. 2-kb transcript is mainly expressed in the brain, with the highest levels in the frontal lobe, cerebral cortex, caudate nucleus, amygdala, temporal lobe, putamen, substantia nigra, and hippocampus. This high-level expression of hSLP-1 in the basal ganglia may also reflect the evolutionary link to UNC-24.

Neurobiology of the Caenorhabditis elegans genome

Neurotransmitter receptors, neurotransmitter synthesis and release pathways, and heterotrimeric GTP-binding protein (G protein)-coupled second messenger pathways are highly conserved between Caenorhabditis elegans and mammals, but gap junctions and chemosensory receptors have independent origins in vertebrates and nematodes. Most ion channels are similar to vertebrate channels but there are no predicted voltage-activated sodium channels. The C. elegans genome encodes at least 80 potassium channels, 90 neurotransmitter-gated ion channels, 50 peptide receptors, and up to 1000 orphan receptors that may be chemoreceptors. For many gene families, C. elegans has both conventional members and divergent outliers with weak homology to known genes; these outliers may provide insights into previously unknown functions of conserved protein families.

Genomewide search for dehydrated hereditary stomatocytosis (hereditary xerocytosis): mapping of locus to chromosome 16 (16q23-qter)

Dehydrated hereditary stomatocytosis, also known as "hereditary xerocytosis," is caused by a red blood cell-membrane defect characterized by stomatocytic morphology, increased mean corpuscular hemoglobin concentration, decreased osmotic fragility, increased permeability to the univalent cations Na+ and K+, and an increased proportion of phosphatidylcholine in the membrane. The clinical presentation is heterogeneous, ranging from mild to moderate hemolytic anemia associated with scleral icterus, splenomegaly, and choletithiasis. Iron overload may develop later in life. The disease is transmitted as an autosomal dominant trait. We recruited a large three-generation Irish family affected with DHS and comprising 23 members, of whom 14 were affected and 9 were healthy. Two additional, small families also were included in the study. The DNA samples from the family members were used in a genomewide search to identify, by linkage analysis, the DHS locus. After the exclusion of a portion of the human genome, we obtained conclusive evidence for linkage of DHS to microsatellite markers on the long arm of chromosome 16 (16q23-q24). A maximum two-point LOD score of 6.62 at recombination fraction .00 was obtained with marker D16S520. There are no recombination events defining the telomeric limit of the region, which therefore is quite large. No candidate genes map to this area.

Unc-1: a stomatin homologue controls sensitivity to volatile anesthetics in Caenorhabditis elegans

To identify sites of action of volatile anesthetics, we are studying genes in a functional pathway that controls sensitivity to volatile anesthetics in the nematode Caenorhabditis elegans. The unc-1 gene occupies a central position in this pathway. Different alleles of unc-1 have unique effects on sensitivity to the different volatile anesthetics. UNC-1 shows extensive homology to human stomatin, an integral membrane protein thought to regulate an associated ion channel. We postulate that UNC-1 has a direct effect on anesthetic sensitivity in C. elegans and may represent a molecular target for volatile anesthetics.

Oligomeric nature of the integral membrane protein stomatin

The 31-kDa integral membrane protein stomatin (protein 7.2b) is not only an important component of the red cell membrane but can also be found in abundance in different tissues and cell lines. The protein is thought to be anchored to the membrane by a hydrophobic domain while both N and C termini are exposed to the cytoplasm. We have previously shown in the human cell line UAC that stomatin concentrates preferentially in plasma membrane folds and protrusions. There is also evidence that stomatin is linked to the cortical actin cytoskeleton, suggesting a role in cortical morphogenesis of the cell. In this study, we demonstrate that the fundamental structure of stomatin is oligomeric. Whereas interaction of stomatin with itself was suggested by cross-linking experiments, we show by density gradient centrifugation analysis that soluble homo-oligomeric complexes of this protein are present in Triton X-100 extracts of UAC cells. We also show the existence of these oligomers by co-immunoprecipitation of the endogenous stomatin and a recombinantly expressed myc-tagged stomatin, using an anti-myc antibody. The data indicate that these complexes comprise between 9 and 12 monomers of stomatin. Two C-terminally truncated forms of stomatin do not incorporate into these oligomers, suggesting an involvement of the C terminus in the homo-oligomeric interaction.

The Groucho-like transcription factor UNC-37 functions with the neural specificity gene unc-4 to govern motor neuron identity in C. elegans

Groucho and Tup1 are members of a conserved family of WD repeat proteins that interact with specific transcription factors to repress target genes. Here we show that mutations in WD domains of the Groucho-like protein, UNC-37, affect a motor neuron trait that also depends on UNC-4, a homeodomain protein that controls neuronal specificity in Caenorhabditis elegans. In unc-4 mutants, VA motor neurons assume the pattern of synaptic input normally reserved for their lineal sister cells, the VB motor neurons; the loss of normal input to the VAs produces a distinctive backward movement defect. Substitution of a conserved residue (H to Y) in the fifth WD repeat in unc-37(e262) phenocopies the Unc-4 movement defect. Conversely, an amino acid change (E to K) in the sixth WD repeat of UNC-37 is a strong suppressor of unc-37(e262) and of specific unc-4 missense mutations. We have previously shown that UNC-4 expression in the VA motor neurons specifies the wild-type pattern of presynaptic input. Here we demonstrate that UNC-37 is also expressed in the VAs and that unc-37 activity in these neurons is sufficient to restore normal movement to unc-37(e262) animals. We propose that UNC-37 and UNC-4 function together to prevent expression of genes that define the VB pattern of synaptic inputs and thereby generate connections specific to the VA motor neurons. In addition, we show that the WD repeat domains of UNC-37 and of the human homolog, TLE1, are functionally interchangeable in VA motor neurons which suggests that this highly conserved protein domain may also specify motor neuron identity and synaptic choice in more complex nervous systems.

A second isoform of 3-ketoacyl-CoA thiolase found in Caenorhabditis elegans, which is similar to sterol carrier protein x but lacks the sequence of sterol carrier protein 2

We cloned a full-length cDNA of the nematode Caenorhabditis elegans that encodes a 44-kDa protein (P-44, 412 residues) similar to sterol carrier protein x (SCPx). Mammalian SCPx is a bipartite protein: its 404-residue N-terminal and 143-residue C-terminal domains are similar to 3-ketoacyl-CoA thiolase and identical to the precursor of sterol carrier protein 2 (SCP2; also termed non-specific lipid-transfer protein), respectively. P-44 has 56% sequence identity to the thiolase domain of SCPx but lacks the SCP2 sequence. Northern blot analysis revealed only a single mRNA species of 1.4 kb, which agrees well with the length of the cDNA (1371 bp), making it improbable that alternative splicing produces an SCPx-like fusion protein. The sequence similarities of P-44 to conventional thiolases are lesser than that to SCPx. Purified recombinant P-44 cleaved long-chain 3-ketoacyl-CoAs (C(8-16)) in a thiolytic manner by the ping-pong bi-bi reaction mechanism. The inhibition of P-44 by acetyl-CoA was competitive with CoA and non-competitive with 3-ketooctanoyl-CoA. This pattern of inhibition is shared with SCPx but not with conventional 3-ketoacyl-CoA thiolase, which is inhibited uncompetitively with respect to 3-ketoacyl-CoA. From these results, we concluded that nematode P-44 and mammalian SCPx constitute a second isoform of thiolase, which we propose to term type-II 3-ketoacyl-CoA thiolase.

Structural and functional conservation of the Caenorhabditis elegans timing gene clk-1

Mutations in the Caenorhabditis elegans gene clk-1 affect biological timing and extend longevity. The gene clk-1 was identified, and the cloned gene complemented the clk-1 phenotypes and restored normal longevity. The CLK-1 protein was found to be conserved among eukaryotes, including humans, and structurally similar to the yeast metabolic regulator Cat5p (also called Coq7p). These proteins contain a tandem duplication of a core 82-residue domain. clk-1 complemented the phenotype of cat5/coq7 null mutants, demonstrating that clk-1 and CAT5/COQ7 share biochemical function and that clk-1 acts at the level of cellular physiology.