ABSTRACT
The Golgi complex is present in every eukaryotic cell and functions in posttranslational modifications and sorting of proteins and lipids to post-Golgi destinations. Both functions require an acidic lumenal pH and transport of substrates into and by-products out of the Golgi lumen. Endogenous ion channels are expected to be important for these features, but none has been described. Ion channels from an enriched Golgi fraction cleared of transiting proteins were incorporated into planar lipid bilayers. Eighty percent of the single-channel recordings revealed the same anion channel. This channel has novel properties and has been named GOLAC (Golgi anion channel). The channel has six subconductance states with a maximum conductance of 130 pS, is open over 95% of the time, and is not voltage-gated. Significant for Golgi function, the channel conductance is increased by reduction of pH on the lumenal surface. This channel may serve two nonexclusive functions: providing counterions for the acidification of the Golgi lumen by the H(+)-ATPase and removal of inorganic phosphate generated by glycosylation and sulfation of proteins and lipids in the Golgi.
Subject(s)
Golgi Apparatus/physiology , Ion Channels/physiology , Lipid Bilayers , 4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid/pharmacology , Animals , Chlorides/pharmacology , Golgi Apparatus/ultrastructure , Liver/ultrastructure , Membrane Potentials/drug effects , Phosphatidylethanolamines , Phosphatidylserines , Potassium Chloride/pharmacology , RatsABSTRACT
To characterize endogenous molecules and activities of the Golgi complex, proteins in transit were > 99% cleared from rat hepatocytes by using cycloheximide (CHX) treatment. The loss of proteins in transit resulted in condensation of the Golgi cisternae and stacks. Isolation of a stacked Golgi fraction is equally efficient with or without proteins in transit [control (CTL SGF1) and cycloheximide (CHX SGF1)]. Electron microscopy and morphometric analysis showed that > 90% of the elements could be positively identified as Golgi stacks or cisternae. Biochemical analysis showed that the cis-, medial-, trans-, and TGN Golgi markers were enriched over the postnuclear supernatant 200- to 400-fold with and 400- to 700-fold without proteins in transit. To provide information on a mechanism for import of calcium required at the later stages of the secretory pathway, calcium uptake into CTL SGF1 and CHX SGF1 was examined. All calcium uptake into CTL SGF1 was dependent on a thapsigargin-resistant pump not resident to the Golgi complex and a thapsigargin-sensitive pump resident to the Golgi. Experiments using CHX SGF1 showed that the thapsigargin-resistant activity was a plasma membrane calcium ATPase isoform in transit to the plasma membrane and the thapsigargin-sensitive pump was a sarcoplasmic/endoplasmic reticulum calcium ATPase isoform. In vivo both of these calcium ATPases function to maintain millimolar levels of calcium within the Golgi lumen.
Subject(s)
Calcium/pharmacokinetics , Golgi Apparatus/metabolism , Golgi Apparatus/ultrastructure , Proteins/metabolism , Adenosine Triphosphatases/antagonists & inhibitors , Adenosine Triphosphate/pharmacology , Animals , Biomarkers/analysis , Calcium/antagonists & inhibitors , Calcium-Transporting ATPases/metabolism , Cell Compartmentation , Cell Membrane/enzymology , Cell Membrane/ultrastructure , Cycloheximide/pharmacology , Data Interpretation, Statistical , Endoplasmic Reticulum/chemistry , Endoplasmic Reticulum/ultrastructure , Enzyme Inhibitors/pharmacology , Golgi Apparatus/drug effects , Immunoblotting , Intracellular Membranes/drug effects , Intracellular Membranes/metabolism , Intracellular Membranes/ultrastructure , Proteins/drug effects , Rats , Sarcoplasmic Reticulum/enzymology , Tissue DistributionABSTRACT
Isolated sagittal vertebral body fractures are rare and the plain radiographic diagnosis may be difficult. We report two cases, confirmed by CT, and describe the subtle MRI features which comprised increased signal intensity only on the midline sagittal T2 weighted images. In one case, the information from MRI significantly altered the patient's management by leading to a change from surgical to conservative treatment.
