Immunohistochemical analysis of the distribution of the human ATPase (hASNA-I) in normal tissues and its overexpression in breast adenomas and carcinomas

Nucleotide-dependent mechanism of Get3 as elucidated from free energy calculations

The unique topology of tail-anchored (TA) proteins precludes them from utilizing the well-studied cotranslational translocation mechanism of most transmembrane proteins, forcing them into a distinct, posttranslational pathway. In yeast, this process is the guided entry of TA-proteins (GET) pathway, which utilizes a combination of cytosolic and transmembrane proteins to identify a TA protein, transfer it, and insert it into the endoplasmic reticulum membrane. At the center of this mechanism is the Get3 homodimer, which transfers a TA protein between the two GET phases by leveraging energy gained in ATP binding and hydrolysis to undergo significant structural changes from "open" to "closed" conformations. We present all-atom molecular dynamics simulations of Get3 in multiple nucleotide states, and through rigorous potential of mean force calculations, compute the free energy landscape of the Get3 opening/closing pathway. Results agree well with experiments on the nucleotide bias of Get3 open and closed structures in the crystallographically observed no-nucleotide, two ATP, and two ADP states, and also reveal their populations in the asymmetric one ATP and one ADP cases. Structures also compare well with the recently observed "semiopen" conformation and suggest that Get3 may sample this state free in solution and not just when bound to Get1, as observed in experiments. Finally, we present evidence for a unique, "wide-open" conformation of Get3. These calculations describe the nucleotide-dependent thermodynamics of Get3 in solution, and improve our understanding of its mechanism in each phase of the GET cycle.

Generation of mice with conditional null allele for GdX/Ubl4A

GdX (also named Ubl4A) is a house-keeping gene located on the X chromosome and encodes a protein harboring an ubiquitin-like domain in human and mouse. Although identified in 1988, the function of GdX remains unknown. To elucidate the role of GdX in vivo, we generated a conditional GdX knockout mouse in which Exon 2 was flanked by two loxP sites. We obtained viable and fertile mice with homozygous GdX(flox/flox) or GdX(flox/Y) allele. Germ-line transmission was confirmed by crossing the mouse bearing conditionally targeted allele with an EIIα-Cre transgenic mouse. GdX was successfully depleted in tissues of EIIα-Cre-GdX-null mice. GdX(-/-) and GdX(-/Y) mice are viable and exhibit normal development compared with wild-type littermates within 6 months during our observation. We also observed that GdX knockout male mice were functionally normal in the reproductive system where Ubl4B was specifically expressed. GdX(flox/flox) and GdX(flox/Y) conditional mice provide a tool for further tissue-specific function analysis of the GdX protein under different conditions.

ASNA-1 activity modulates sensitivity to cisplatin

Cancer can be cured by platinum-based chemotherapy, but resistance is a major cause of treatment failure. Here we present the nematode Caenorhabditis elegans as a model to study interactions between the platinum drug cisplatin and signaling pathways in vivo. Null mutation in a single gene, asna-1, makes worms hypersensitive to cisplatin. The metalloregulated ATPase ASNA-1 promotes insulin secretion and membrane insertion of tail-anchored proteins. Using structural data from ASNA-1 homologues, we identify specific ASNA-1 mutants that are sensitive to cisplatin while still able to promote insulin signaling. Mutational analysis reveals that hypersensitivity of ASNA-1 mutants to cisplatin remains in absence of CEP-1/p53 or apoptosis. Human ASNA1 can substitute for the worm gene, indicating a conserved function. Cisplatin sensitivity is not affected by decreased insulin signaling in wild-type nematodes or restored insulin signaling in asna-1 mutants. These findings provide a functional insight into ASNA-1, demonstrate that C. elegans can be used to characterize cisplatin resistance mechanisms, and suggest that rationally designed drugs against ASNA-1 can sensitize cancer cells to cisplatin.

Structures of Get3, Get4, and Get5 provide new models for TA membrane protein targeting

The GET pathway, using several proteins (Gets 1-5 and probably Sgt2), posttranslationally conducts tail-anchored (TA) proteins to the endoplasmic reticulum (ER). At the ER, TA proteins are inserted into the lipid bilayer and then sorted and directed to their respective destinations in the secretory pathway. Until last year, there was no structural information on any of the GET components but now there are ten crystal structures of Get3 in a variety of nucleotide-bound states and conformations. The structures of Get4 and a portion of Get5 also emerged in 2010. This minireview provides a detailed comparison of the GET structures and discusses their mechanistic relevance to TA protein insertion. It also addresses the outstanding gaps in detailed molecular information on this system, including the structures of Get5, Sgt2, and the transmembrane complex comprising Get1 and Get2.

ASNA1, an ATPase targeting tail-anchored proteins, regulates melanoma cell growth and sensitivity to cisplatin and arsenite

PURPOSE

ASNA1 is homologous to E. coli ArsA, a well characterized ATPase involved in efflux of arsenite and antimonite. Cells resistant to arsenite and antimonite are cross-resistant to the chemotherapeutic drug cisplatin. ASNA1 is also an essential ATPase for the insertion of tail-anchored proteins into ER membranes and a novel regulator of insulin secretion. The aim of this study was to determine if altered ASNA1 levels influenced growth and sensitivity to arsenite and cisplatin in human melanoma cells.

METHODS

Cultured melanoma T289 cells were transfected with plasmids containing sense or antisense ASNA1. Cells were exposed to cisplatin, arsenite and zinc. Cell growth and chemosensitivity were evaluated by the MTT assay and apoptosis by a TUNEL assay.

RESULTS

ASNA1 expression was necessary for growth. T289 clones with decreased ASNA1 expression exhibited 51 +/- 5% longer doubling times than wildtype T289 (P = 0.0091). After exposure to cisplatin, ASNA1 downregulated cells displayed a significant increase in apoptosis. The cisplatin IC(50) in ASNA1 underexpressing cells was 41.7 +/- 1.8% compared to wildtype (P = 0.00097) and the arsenite IC(50) was 59.9 +/- 3.2% of wildtype IC(50) (P = 0.0067).

CONCLUSIONS

Reduced ASNA1 expression is associated with significant inhibition of cell growth, increased apoptosis and increased sensitivity to cisplatin and arsenite.

Distinct targeting pathways for the membrane insertion of tail-anchored (TA) proteins

Tail-anchored (TA) proteins are characterised by a C-terminal transmembrane region that mediates post-translational insertion into the membrane of the endoplasmic reticulum (ER). We have investigated the requirements for membrane insertion of three TA proteins, RAMP4, Sec61beta and cytocrome b5. We show here that newly synthesised RAMP4 and Sec61beta can accumulate in a cytosolic, soluble complex with the ATPase Asna1 before insertion into ER-derived membranes. Membrane insertion of these TA proteins is stimulated by ATP, sensitive to redox conditions and blocked by alkylation of SH groups by N-ethylmaleimide (NEM). By contrast, membrane insertion of cytochrome b5 is not found to be mediated by Asna1, not stimulated by ATP and not affected by NEM or an oxidative environment. The Asna1-mediated pathway of membrane insertion of RAMP4 and Sec61beta may relate to functions of these proteins in the ER stress response.

ASNA-1 positively regulates insulin secretion in C. elegans and mammalian cells

C. elegans worms hatching in the absence of food show growth arrest during the first larval stage (L1). While much has been learned about the later diapause, dauer, which worms enter under adverse conditions, much less is known about the mechanisms governing L1 arrest. Here we show that worms lacking activity of the asna-1 gene arrest growth reversibly at the L1 stage even when food is abundant. asna-1 encodes an ATPase that functions nonautonomously to regulate growth. asna-1 is expressed in a restricted set of sensory neurons and in insulin-producing intestinal cells. asna-1 mutants are reduced in insulin secretion while overexpression of asna-1 mimics the effects of insulin overexpression. Human ASNA1 is highly expressed in pancreatic beta cells, but not in other pancreatic endocrine cell types, and regulates insulin secretion in cultured cells. We propose that ASNA1 is an evolutionarily conserved modulator of insulin signaling.