The Mammalian Calcium-binding Protein, Nucleobindin (CALNUC), Is a Golgi Resident Protein

We have identified CALNUC, an EF-hand, Ca²⁺-binding protein, as a Golgi resident protein. CALNUC corresponds to a previously identified EF-hand/calcium-binding protein known as nucleobindin. CALNUC interacts with Gα_i3 subunits in the yeast two-hybrid system and in GST-CALNUC pull-down assays. Analysis of deletion mutants demonstrated that the EF-hand and intervening acidic regions are the site of CALNUC's interaction with Gα_i3. CALNUC is found in both cytosolic and membrane fractions. The membrane pool is tightly associated with the luminal surface of Golgi membranes. CALNUC is widely expressed, as it is detected by immunofluorescence in the Golgi region of all tissues and cell lines examined. By immunoelectron microscopy, CALNUC is localized to cis-Golgi cisternae and the cis-Golgi network (CGN). CALNUC is the major Ca²⁺-binding protein detected by ⁴⁵Ca²⁺-binding assay on Golgi fractions. The properties of CALNUC and its high homology to calreticulin suggest that it may play a key role in calcium homeostasis in the CGN and cis-Golgi cisternae.     

The rhythm of retinoids in the brain

The retinoids are a family of compounds that in nature are derived from vitamin A or pro‐vitamin A carotenoids. An essential part of the diet for mammals, vitamin A has long been known to be essential for many organ systems in the adult. More recently, however, they have been shown to be necessary for function of the brain and new discoveries point to a central role in processes ranging from neuroplasticity to neurogenesis. Acting in several regions of the central nervous system including the eye, hippocampus and hypothalamus, one common factor in its action is control of biological rhythms. This review summarizes the role of vitamin A in the brain; its action through the metabolite retinoic acid via specific nuclear receptors, and the regulation of its concentration through controlled synthesis and catabolism. The action of retinoic acid to regulate several rhythms in the brain and body, from circadian to seasonal, is then discussed to finish with the importance of retinoic acid in the regular pattern of sleep.

     

Mitochondrial inner-membrane fusion and crista maintenance requires the dynamin-related GTPase Mgm1

Mitochondrial outer- and inner-membrane fusion events are coupled in vivo but separable and mechanistically distinct in vitro, indicating that separate fusion machines exist in each membrane. Outer-membrane fusion requires trans interactions of the dynamin-related GTPase Fzo1, GTP hydrolysis, and an intact inner-membrane proton gradient. Inner-membrane fusion also requires GTP hydrolysis but distinctly requires an inner-membrane electrical potential. The protein machinery responsible for inner-membrane fusion is unknown. Here, we show that the conserved intermembrane-space dynamin-related GTPase Mgm1 is required to tether and fuse mitochondrial inner membranes. We observe an additional role of Mgm1 in inner-membrane dynamics, specifically in the maintenance of crista structures. We present evidence that trans Mgm1 interactions on opposing inner membranes function similarly to tether and fuse inner membranes as well as maintain crista structures and propose a model for how the mitochondrial dynamins function to facilitate fusion.     

UCP2 regulates energy metabolism and differentiation potential of human pluripotent stem cells

It has been assumed, based largely on morphologic evidence, that human pluripotent stem cells (hPSCs) contain underdeveloped, bioenergetically inactive mitochondria. In contrast, differentiated cells harbour a branched mitochondrial network with oxidative phosphorylation as the main energy source. A role for mitochondria in hPSC bioenergetics and in cell differentiation therefore remains uncertain. Here, we show that hPSCs have functional respiratory complexes that are able to consume O(2) at maximal capacity. Despite this, ATP generation in hPSCs is mainly by glycolysis and ATP is consumed by the F(1)F(0) ATP synthase to partially maintain hPSC mitochondrial membrane potential and cell viability. Uncoupling protein 2 (UCP2) plays a regulating role in hPSC energy metabolism by preventing mitochondrial glucose oxidation and facilitating glycolysis via a substrate shunting mechanism. With early differentiation, hPSC proliferation slows, energy metabolism decreases, and UCP2 is repressed, resulting in decreased glycolysis and maintained or increased mitochondrial glucose oxidation. Ectopic UCP2 expression perturbs this metabolic transition and impairs hPSC differentiation. Overall, hPSCs contain active mitochondria and require UCP2 repression for full differentiation potential.     

The Assembly Domain of the Small Capsid Protein of Kaposi's Sarcoma-Associated Herpesvirus

Self-assembly of Kaposi''s sarcoma-associated herpesvirus capsids occurs when six proteins are coexpressed in insect cells using recombinant baculoviruses; however, if the small capsid protein (SCP) is omitted from the coinfection, assembly does not occur. Herein we delineate and identify precisely the assembly domain and the residues of SCP required for assembly. Hence, six residues, R14, D18, V25, R46, G66, and R70 in the assembly domain, when changed to alanine, completely abolish or reduce capsid assembly.     

Aldehyde dehydrogenase is a positional marker in the retina

An asymmetrically distributed protein in the embryonic mouse retina was identified as an aldehyde dehydrogenase through protein microsequencing. It was characterized as a cytosolic isoform with basic isoelectric point and preference for aliphatic substrates, features that resemble those of the isoform AHD-2 which is known to oxidize retinaldehyde to retinoic acid. Immunohistochemistry with aldehyde dehydrogenase antisera showed strong labeling of the dorsal retina from the early eye vesicle stage into adulthood. In addition, optic axons originating from the dorsal retina were transiently labeled during their outgrowth phase. Whereas in the embryo the enzyme was expressed in undifferentiated cells and in neurons, in the retina of the adult mouse the asymmetrically distributed isoform was mainly expressed in Müller glia, with the number of labeled glial cells varying with retinal position.