In mammals and birds, thermoregulation to conserve body temperature is vital to life. Multiple mechanisms of thermogeneration have been proposed, localized in different subcellular organelles. However, visualizing thermogenesis directly in intact organelles has been challenging. Here we have developed genetically encoded, GFP-based thermosensors (tsGFPs) that enable visualization of thermogenesis in discrete organelles in living cells. In tsGFPs, a tandem formation of coiled-coil structures of the Salmonella thermosensing protein TlpA transmits conformational changes to GFP to convert temperature changes into visible and quantifiable fluorescence changes. Specific targeting of tsGFPs enables visualization of thermogenesis in the mitochondria of brown adipocytes and the endoplasmic reticulum of myotubes. In HeLa cells, tsGFP targeted to mitochondria reveals heterogeneity in thermogenesis that correlates with the electrochemical gradient. Thus, tsGFPs are powerful tools to noninvasively assess thermogenesis in living cells.

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The vesicle-associated membrane proteins [VAMPs; vesicle SNAP receptors (v-SNAREs)] present on GLUT4-enriched vesicles prepared from rat adipose cells [Cain, Trimble and Lienhard (1992) J. Biol. Chem. 267, 11681-11684] have been identified as synaptobrevin 2 (VAMP 2) and cellubrevin (VAMP 3) by using isoform-specific antisera. Additional antisera identify syntaxins 2 and 4 as the predominant target membrane SNAP receptors (t-SNAREs) in the plasma membranes (PM), with syntaxin 3 at one-twentieth the level. Syntaxins 2 and 4 are enriched 5-10-fold in PM compared with low-density microsomes (LDM). Insulin treatment results in an 11-fold increase in immunodetectable GLUT4 in PM and smaller (approx. 2-fold) increases in VAMP 2 and VAMP 3, whereas the subcellular distributions of the syntaxins are not altered by insulin treatment. To determine which of the SNAP receptors (SNAREs) in PM might participate in SNARE complexes with proteins from GLUT4 vesicles, complexes were immunoprecipitated with anti-myc antibody from solubilized membranes after the addition of myc-epitope-tagged N-ethylmaleimide-sensitive fusion protein (NSF) and recombinant alpha-soluble NSF attachment protein (alpha-SNAP). These complexes contain VAMPs 2 and 3 and syntaxin 4, but not syntaxins 2 or 3. Complex formation requires ATP and is disrupted by ATP hydrolysis. When all membrane fractions are prepared from basal cells, few or no VAMPs and no syntaxin 4 are immunoprecipitated in SNARE complexes obtained from LDM alone (or from immunoisolated GLUT4 vesicles). The content of syntaxin 4 depends on the presence of PM, and participation of VAMPs 2 and 3 is enhanced 4-6-fold by the addition of solubilized GLUT4 vesicles to PM. The latter increase is greater than can be explained by the 2-fold higher levels of VAMPs added to the reaction mixture. When all membrane fractions are prepared from insulin-stimulated cells, SNARE complexes formed from PM alone contain similar levels of syntaxin 4 but 5-6-fold higher levels of VAMPs 2 and 3 compared with PM alone from basal cells. Addition of GLUT4 vesicle proteins to PM from insulin-treated cells results in a further 2-fold increase in VAMP 2 recovered in SNARE complexes. Therefore the VAMPs in PM of insulin-treated but not basal cells, and in GLUT4-vesicles from cells in either condition, are in a form that readily forms a SNARE complex with PM t-SNAREs and NSF. Insulin seems to activate PM and/or GLUT4 vesicles so as to increase the efficiency of SNARE complex formation.

1 The present investigation was undertaken to characterize the Na + /K + pump current in small (p25 mm in soma diameter) dorsal root ganglion (DRG) neurons isolated from lumbar L4-6 segments of adult rats. 2 The Na + /K + pump current was identified as an ouabain-sensitive current during square voltage steps to membrane potentials between +40 and À120 mV, using the whole-cell patch-clamp technique in which Ca 2+ and K + channel currents and Na + /Ca 2+ exchange currents were minimized. The Na + / K + pump current was practically time-independent over the entire voltage range examined and exhibited a voltage-dependence; its current -voltage (I-V) relationship displayed a positive slope at potentials between À120 and 0 mV but nearly plateau levels at positive membrane potentials. 3 The concentration-dependent block of Na + /K + pump current (activated by 30 mm pipette Na + ) by ouabain at concentrations between 0.1 mm and 5 mm was biphasic and was well described using a two-binding site model with dissociation constants for high-and low-affinity binding sites of 0.20 and 140.1 mm, respectively. The relative amplitude of the Na + /K + pump current produced by low-and high-affinity sites (probably a1b1 and a3b1 isozymes, respectively) was estimated to be 13 : 1 in the presence of 30 mm Na + in the pipette solution. 4 Additionally, the activation of Na + /K + pump current by pipette Na + at concentrations ranging from 5 to 100 mm also exhibited a biphasic concentration dependence which can be reasonably well fitted by assuming the existence of two isozymes having high and low affinities for Na + (6.7 and 67.6 mm, respectively). 5 Thus, the present investigation provides functional evidence to suggest that the Na + /K + ATPase comprises two functionally distinct isozymes as expected for a1b1 and a3b1 in rat small DRG neurons.