NEWS VIEWS NATURE Vol 450 13 December 2007 P A DUC CEA CNRS NRAO AUI NSF NASA they had been assumed to be This means that hierarchical galaxy formation theory within a CDM model is not required to yield GLSBs as final products the challenge they represent to the theory vanishes In fact the best fit models of Mapelli et al all include haloes of CDM surrounding the colliding galaxies Sound as it might like a fractured fairy tale or a Wagner opera gone wrong the evidence seems to suggest that cosmic gentle giants spring from bejewelled rings But mysteries remain such as how smaller gas rich lowsurface brightness galaxies form and how they persist so quietly until they are lit up by a collision With rapid advances in our understanding of galaxy formation the prospects are bright for a speedy resolution Curtis Struck is in the Department of Physics and Astronomy Iowa State University Ames Iowa 50011 USA e mail curt iastate edu 1 Pickering T E Impey C D van Gorkom J H Bothun G D Astron J 114 1858 1882 1997 2 Impey C Bothun G Astrophys J 341 89 104 1989 3 Edmunds M G Nature 341 105 106 1989 4 Mapelli M et al Mon Not R Astron Soc in the press preprint at www arxiv org abs 0710 5354 2007 5 Springel V Frenk C S White S D M Nature 440 1137 1144 2006 6 Springel V et al Nature 435 629 636 2005 7 Governato F et al Mon Not R Astron Soc 374 1479 1494 2007 8 Lynds R Toomre A Astrophys J 209 382 388 1976 9 Appleton P N Struck Marcell C Fund Cosmic Phys 16 111 220 1996 10 Bournaud F et al Science 316 1166 1169 2007 STRUCTURAL BIOLOGY Ion pumps made crystal clear Figure 2 Collision remnant A multi wavelength image of the hyper extended ring galaxy NGC 5291 observation9 especially of systems where the companion is somewhat less massive than the primary disk and is in or near a gravitationally bound orbit Recent extensions have taken into account observations of several extreme processes among them high velocity collisions between galaxies that fall together from distant points but that are both bound to a greater structure such as a galaxy group or cluster The interacting system NGC 5291 ref 10 is a prominent example of the hyperextended rings that can result Fig 2 In large rings the gas density is likely to be low so the star formation rate will also be low Mapelli and colleagues central idea is that such a weak ring will generally be so dim as to be barely visible and that the huge disk that it leaves behind it as it propagates outwards will look remarkably like a GLSB Moreover in the time a billion years or so that it takes the material to travel out to such large distances the high speed companion will in many cases have moved off the immediate scene It is therefore hardly surprising if we see no obvious evidence of the past fracas The authors4 use extensive data on four GLSBs to select from a grid of computer simulations with a range of initial collision parameters those models and the times during their evolution that are most like the observed systems In all four cases a model is found in which the surface brightness profiles optical colours and the gas distributions and kinematics of the simulated galaxy all agree well with observations In three out of four cases possible companions are also in view The obvious consequence of the hypothesis is that GLSBs are disturbed bodies and not the stable quiescent galaxy disks that David C Gadsby The function of every cell in our bodies depends on the work of proteins known as ion pumps Several new crystal structures cast fresh light on how three different pumps deal with their distinct cargoes of ions Ion pumps toil tirelessly in cells throughout all kingdoms of life transporting ions across membranes To investigate the workings of these microscopic machines X ray crystal structures of a calcium ion pump known as SERCA have been determined1 5 But although those structures depict SERCA in several conformations none of them caught the pump in the act of releasing its cargo of ions Moreover nagging questions remained about how much SERCA might differ from other genetically related ion pumps such as those that transport ions of different sizes and charges from calcium or that require additional protein subunits In this issue three papers6 8 from the same group go a long way towards addressing those concerns by describing the first atomic structures of a SERCA pump with its ion pathway open6 and of two related proteins a sodium potassium pump7 and a proton pump8 The three pumps described in these papers belong to a family known as phosphorylated type P type pumps named after the phosphate group whose addition and removal controls their activity P type pumps inhabit all our cells and are essential for life Without sodium potassium pumps many vital functions would fail For example there would be no electrical signals in our brains or hearts or in any nerves or muscles and without SERCA pumps there would be no muscle contraction Not surprisingly P type pumps are hot targets for therapeutics for example digoxin a treatment for heart problems targets sodium potassium pumps and the latest antacids act on the proton potassium pumps in our stomachs So the stakes are high determining the structure and mechanism of each P type pump is crucial for further drug discovery P type pumps reside either in the surface membranes of cells or in the membranes of intracellular organelles such as the endoplasmic or sarcoplasmic reticulum In all cases one end of the pump opens to the cytoplasm and the other end opens either to the outside of the cell or to the interior lumen of the organelle The pumps adopt two main conformations9 known as E1 and E2 Fig 1 overleaf The ion binding sites are found deep inside the region of the pump that crosses the membrane in E1 these sites are accessible to ions in the cytoplasm Ion binding promotes the phosphorylation of the pump in which a phosphate group is added to a single amino acid residue The source of the phosphate is an ATP molecule a side product ADP is formed that briefly remains associated with the pump In the resulting E1P state the bound ions are occluded they are inaccessible from either side of the membrane The pump then releases the ADP and relaxes to the E2P conformation whereupon a pathway opens to the extra cytoplasmic side allowing the ions to escape Transport in the reverse direction begins when ions from the cell exterior or the sarcoplasmic reticulum bind to the exposed binding sites in the E2P state triggering