RESUMEN
Five yeast enzymes synthesizing various glycerophospholipids belong to the CDP-alcohol phosphatidyltransferase (CAPT) superfamily. They only share the so-called CAPT motif, which forms the active site of all these enzymes. Bioinformatic tools predict the CAPT motif of phosphatidylinositol synthase Pis1 as either ER luminal or cytosolic. To investigate the membrane topology of Pis1, unique cysteine residues were introduced into either native or a Cys-free form of Pis1 and their accessibility to the small, membrane permeating alkylating reagent N-ethylmaleimide (NEM) and mass tagged, non-permeating maleimides, in the presence and absence of non-denaturing detergents, was monitored. The results clearly point to a cytosolic location of the CAPT motif. Pis1 is highly sensitive to non-denaturing detergent, and low concentrations (0.05%) of dodecylmaltoside change the accessibility of single substituted Cys in the active site of an otherwise cysteine free version of Pis1. Slightly higher detergent concentrations inactivate the enzyme. Removal of the ER retrieval sequence from (wt) Pis1 enhances its activity, again suggesting an influence of the lipid environment. The central 84% of the Pis1 sequence can be aligned and fitted onto the 6 transmembrane helices of two recently crystallized archaeal members of the CAPT family. Results delineate the accessibility of different parts of Pis1 in their natural context and allow to critically evaluate the performance of different cysteine accessibility methods. Overall the results show that cytosolically made inositol and CDP-diacylglycerol can access the active site of the yeast PI synthase Pis1 from the cytosolic side and that Pis1 structure is strongly affected by mild detergents.
Asunto(s)
CDP-Diacilglicerol-Inositol 3-Fosfatidiltransferasa/metabolismo , Citosol/enzimología , Saccharomyces cerevisiae/enzimología , Transferasas (Grupos de Otros Fosfatos Sustitutos)/metabolismo , Algoritmos , CDP-Diacilglicerol-Inositol 3-Fosfatidiltransferasa/química , CDP-Diacilglicerol-Inositol 3-Fosfatidiltransferasa/genética , Dominio Catalítico , Biología Computacional , Cisteína , Citidina Difosfato Diglicéridos/metabolismo , Detergentes/química , Activación Enzimática , Estabilidad de Enzimas , Inositol/metabolismo , Modelos Moleculares , Mutagénesis Sitio-Dirigida , Mutación , Conformación Proteica , Desnaturalización Proteica , Estructura Terciaria de Proteína , Saccharomyces cerevisiae/genética , Saccharomyces cerevisiae/crecimiento & desarrollo , Relación Estructura-Actividad , Especificidad por Sustrato , Factores de Tiempo , Transferasas (Grupos de Otros Fosfatos Sustitutos)/química , Transferasas (Grupos de Otros Fosfatos Sustitutos)/genéticaRESUMEN
Most integral membrane proteins of yeast with two or more membrane-spanning sequences have not yet been crystallized and for many of them the side on which the active sites or ligand-binding domains reside is unknown. Also, bioinformatic topology predictions are not yet fully reliable. However, so-called low-resolution biochemical methods can be used to locate hydrophilic loops or individual residues of polytopic membrane proteins at one or the other side of the membrane. The advantages and limitations of several such methods for topological studies with yeast ER integral membrane proteins are discussed. We also describe new tools that allow us to better control and validate results obtained with SCAM (substituted cysteine accessibility method), an approach that determines the position of individual residues with respect to the membrane plane, whereby only minimal changes in the primary sequence have to be introduced into the protein of interest.
Asunto(s)
Proteínas de la Membrana/metabolismo , Microsomas/metabolismo , Proteínas de Saccharomyces cerevisiae/metabolismo , Saccharomyces cerevisiae/metabolismo , Epítopos/metabolismo , GlicosilaciónRESUMEN
All glycerophospholipids are made from phosphatidic acid, which, according to the traditional view, is generated at the cytosolic surface of the ER. In yeast, phosphatidic acid is synthesized de novo by two acyl-CoA-dependent acylation reactions. The first is catalysed by one of the two homologous glycerol-3-phosphate acyltransferases Gpt2p/Gat1p and Sct1p/Gat2p, the second by one of the two 1-acyl-sn-glycerol-3-phosphate acyltransferases Slc1p and Ale1p/Slc4p. To study the biogenesis and topology of Gpt2p we observed the location of dual topology reporters inserted after various transmembrane helices. Moreover, using microsomes, we probed the accessibility of natural and substituted cysteine residues to a membrane impermeant alkylating agent and tested the protease sensitivity of various epitope tags inserted into Gpt2p. Finally, we assayed the sensitivity of the acyltransferase activity to membrane impermeant agents targeting lysine residues. By all these criteria we find that the most conserved motifs of Gpt2p and its functionally relevant lysines are oriented towards the ER lumen. Thus, the first step in biosynthesis of phosphatidic acid in yeast seems to occur in the ER lumen and substrates may have to cross the ER membrane.
Asunto(s)
Retículo Endoplásmico/metabolismo , Glicerol-3-Fosfato O-Aciltransferasa/metabolismo , Microsomas/enzimología , Ácidos Fosfatidicos/biosíntesis , Saccharomyces cerevisiae/enzimología , Dominio Catalítico , Glicerol-3-Fosfato O-Aciltransferasa/química , Glicerol-3-Fosfato O-Aciltransferasa/genética , Saccharomyces cerevisiae/genética , Proteínas de Saccharomyces cerevisiae/química , Proteínas de Saccharomyces cerevisiae/genética , Proteínas de Saccharomyces cerevisiae/metabolismo , Especificidad por SustratoRESUMEN
In Caenorhabditis elegans, germ cells develop as spermatids in the larva and as oocytes in the adult. Such fundamentally different gametes are produced through a fine-tuned balance between feminizing and masculinizing genes. For example, the switch to oogenesis requires repression of the fem-3 mRNA through the mog genes. Here we report on the cloning and characterization of the sex determination gene mog-2. MOG-2 is the worm homolog of spliceosomal protein U2A'. We found that MOG-2 is expressed in most nuclei of somatic and germ cells. In addition to its role in sex determination, mog-2 is required for meiosis. Moreover, MOG-2 binds to U2Bâ³/RNP-3 in the absence of RNA. We also show that MOG-2 associates with the U2 snRNA in the absence of RNP-3. Therefore, we propose that MOG-2 is a bona fide component of the U2 snRNP. Albeit not being required for general pre-mRNA splicing, MOG-2 increases the splicing efficiency to a cryptic splice site that is located at the 5' end of the exon.