2006年七月《cell》（July 14）整理(plant vision)

1. Cell, Vol 126, 25-27, 14 July 2006
Heterotrimeric G Protein Signaling: Getting inside the Cell
Michael R. Koelle1, (1 Department of Molecular Biophysics & Biochemistry, Yale University, New Haven, CT 06520, USA;Correspondence:Michael R. KoelleCorresponding authormailto:authormichael.koelle@yale.edu)

Heterotrimeric G proteins have traditionally been thought to transduce signals at the plasma membrane. In this issue, Slessareva et al., 2006 now show that a G protein α subunit acts at the endosome to stimulate a phosphatidylinositol 3-kinase to help yeast respond to mating pheromones.


2. Cell, Vol 126, 27-29, 14 July 2006
OPA1 and PARL Keep a Lid on Apoptosis
(Eyal Gottlieb1, 1 Apoptosis and tumour physiology laboratory, Cancer Research UK, The Beatson Institute for Cancer Research, Glasgow, Scotland G61 1BD, UK
Correspondence:Eyal GottliebCorresponding authormailto:authore.gottlieb@beatson.gla.ac.uk)

A change in the shape of mitochondrial cristae must take place to attain rapid and complete release of cytochrome c during apoptosis. In this issue of Cell, Cipolat et al., 2006 and Frezza et al., 2006 show that a rhomboid intramembrane protease PARL and a dynamin-related protein OPA1 are critical regulators of cristae remodeling.

3. Cell, Vol 126, 79-92, 14 July 2006
The Arabidopsis Chromatin-Modifying Nuclear siRNA Pathway Involves a Nucleolar RNA Processing Center
(Olga Pontes,1 Carey Fei Li,2 Pedro Costa Nunes,1,4 Jeremy Haag,1 Thomas Ream,1 Alexa Vitins,1 Steven E. Jacobsen,2,3 and Craig S. Pikaard1,
1 Biology Department, Washington University, 1 Brookings Drive, St. Louis, MO 63130, USA2 Department of Molecular, Cell and Developmental Biology, University of California, Los Angeles, Los Angeles, CA 90095, USA3 Howard Hughes Medical Institute, University of California, Los Angeles, Los Angeles, CA 90095, USA4 Secção de Genética, Centro de Botânica Aplicada à Agricultura, Instituto Superior de Agronomia, Tapada da Ajuda, 1349-017 Lisboa, Portugal
Correspondence:Craig S. PikaardCorresponding authormailto:authorpikaard@biology.wustl.edu)

In Arabidopsis thaliana, small interfering RNAs (siRNAs) direct cytosine methylation at endogenous DNA repeats in a pathway involving two forms of nuclear RNA polymerase IV (Pol IVa and Pol IVb), RNA-DEPENDENT RNA POLYMERASE 2 (RDR2), DICER-LIKE 3 (DCL3), ARGONAUTE4 (AGO4), the chromatin remodeler DRD1, and the de novo cytosine methyltransferase DRM2. We show that RDR2, DCL3, AGO4, and NRPD1b (the largest subunit of Pol IVb) colocalize with siRNAs within the nucleolus. By contrast, Pol IVa and DRD1 are external to the nucleolus and colocalize with endogenous repeat loci. Mutation-induced loss of pathway proteins causes downstream proteins to mislocalize, revealing their order of action. Pol IVa acts first, and its localization is RNA dependent, suggesting an RNA template. We hypothesize that maintenance of the heterochromatic state involves locus-specific Pol IVa transcription followed by siRNA production and assembly of AGO4- and NRPD1b-containing silencing complexes within nucleolar processing centers.


4. Cell, Vol 126, 191-203, 14 July 2006
Activation of the Phosphatidylinositol 3-Kinase Vps34 by a G Protein α Subunit at the Endosome
(Janna E. Slessareva,1 Sheri M. Routt,2 Brenda Temple,1 Vytas A. Bankaitis,2 and Henrik G. Dohlman1, 1 Department of Biochemistry and Biophysics, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA2 Department of Cell Biology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA Correspondence:Henrik G. DohlmanCorresponding authormailto:authorhdohlman@med.unc.edu)

In the yeast Saccharomyces cerevisiae, the G protein βγ subunits are essential for pheromone signaling. The Gα subunit Gpa1 can also promote signaling, but the effectors in this pathway are not well characterized. To identify candidate Gpa1 effectors, we expressed the constitutively active Gpa1Q323L mutant in each of nearly 5000 gene-deletion strains and measured mating-specific responses. Our analysis reveals a requirement for both the catalytic (Vps34) and regulatory (Vps15) subunits of the sole phosphatidylinositol 3-kinase in yeast. We demonstrate that Gpa1 is present at endosomes, where it interacts directly with both Vps34 and Vps15 and stimulates increased production of phosphatidylinositol 3-phosphate. Notably, Vps15 binds to GDP-bound Gpa1 and is predicted to have a seven-WD repeat structure similar to that of known G protein β subunits. These findings reveal two new components of the pheromone signaling pathway. More remarkably, these proteins appear to comprise a preformed effector-Gβ subunit assembly and function at the endosome rather than at the plasma membrane.


5. Cell, Vol 126, 205-218, 14 July 2006
NOX2 Controls Phagosomal pH to Regulate Antigen Processing during Crosspresentation by Dendritic Cells
(Ariel Savina,1 Carolina Jancic,1 Stephanie Hugues,1 Pierre Guermonprez,1 Pablo Vargas,3 Ivan Cruz Moura,1 Ana-Maria Lennon-Duménil,3 Miguel C. Seabra,2 Graça Raposo,4 and Sebastian Amigorena1, 1 Institut Curie, INSERM U653, Immunité et Cancer, 26 rue d'Ulm, 75248 Paris, Cedex 05, France2 Cell and Molecular Biology, Division of Biomedical Sciences, Imperial College London, Exhibition Road, London SW7 2AZ, UK3 Institut Curie, INSERM U520, Biologie Cellulaire de l'Immunité Tumoral, 26 rue d'Ulm, 75248 Paris, Cedex 05, France4 Institut Curie, CNRS-UMR144, Structures et Compartiments Membranaires, 26 rue d'Ulm, 75248 Paris, Cedex 05, France Correspondence:Sebastian AmigorenaCorresponding authormailto:authorsebastian.amigorena@curie.fr)

To initiate adaptative cytotoxic immune responses, proteolytic peptides derived from phagocytosed antigens are presented by dendritic cells (DCs) to CD8+ T lymphocytes through a process called antigen “crosspresentation.” The partial degradation of antigens mediated by lysosomal proteases in an acidic environment must be tightly controlled to prevent destruction of potential peptides for T cell recognition. We now describe a specialization of the phagocytic pathway of DCs that allows a fine control of antigen processing. The NADPH oxidase NOX2 is recruited to the DC's early phagosomes and mediates the sustained production of low levels of reactive oxygen species, causing active and maintained alkalinization of the phagosomal lumen. DCs lacking NOX2 show enhanced phagosomal acidification and increased antigen degradation, resulting in impaired crosspresentation. Therefore, NOX2 plays a critical role in conferring DCs the ability to function as specialized phagocytes adapted to process antigens rather than kill pathogens.