Transport of secretory and membrane glycoproteins from the rough endoplasmic reticulum to the Golgi. A rate-limiting step in protein maturation and secretion

Metalloendoprotease inhibitors block fast axonal transport

Metalloendoprotease activity that was sensitive to the metal chelator 1,10-phenanthroline and to synthetic dipeptide substrates of the enzyme was detected in homogenates of dorsal root ganglia (DRG) and spinal nerve from the bullfrog. Exposure of an intact in vitro preparation of DRG and spinal nerves to 1,10-phenanthroline led to a dose-dependent depression in the accumulation of fast-transported 3H-labeled protein proximal to a nerve ligature. In nonligated preparations, the chelator treatment reduced the amount of transported protein entering the nerve; no marked effect on the transport rate was observed. Exposure of a desheathed region of spinal nerve to 1,10-phenanthroline, while DRG were maintained in control medium, resulted in a slight depression of fast transport. This effect was not dose dependent over the range that produced a dose response when both DRG and spinal nerve were exposed to the drug. Treatment of DRG and spinal nerve with the metalloendoprotease substrate analogues carbobenzoxy (CBZ)-Ser-Leu-amide or CBZ-Gly-Leu-amide inhibited fast axonal transport, whereas treatment with CBZ-Gly-Gly-amide, which is not a substrate, had no detectable effect on transport. Selective exposure of desheathed nerve trunk to CBZ-Ser-Leu-amide inhibited fast transport, but the effect was less marked than when DRG and nerve trunk were treated. Although previous studies have focused on the role of metalloendoprotease activity in exocytosis, the present data suggest that the enzyme may also be involved in earlier stages of intracellular transport.

Repair of the secretion defect in the Z form of alpha 1-antitrypsin by addition of a second mutation

Homozygous inheritance of the Z-type mutant form of the alpha 1-antitrypsin (alpha 1AT) gene results in the most common form of alpha 1AT deficiency, a human hereditary disease associated with a high risk for the development of emphysema and an increased incidence of neonatal hepatitis. The alpha 1AT-synthesizing cells of individuals with the Z gene have normal alpha 1AT messenger RNA levels, but alpha 1AT secretion is markedly reduced secondary to accumulation of newly synthesized alpha 1AT in the rough endoplasmic reticulum. Crystallographic analysis of alpha 1AT predicts that in normal alpha 1AT, a negatively charged Glu342 is adjacent to positively charged Lys290. Thus the Glu342----Lys342 Z mutation caused the loss of a normal salt bridge, resulting in the intracellular aggregation of the Z molecule. The prediction was made that a second mutation in the alpha 1AT genet that changed the positively charged Lys290 to a negatively charged Glu290 would correct the secretion defect. When the second mutation was added to the Z-type complementary DNA, the resulting gene directed the synthesis and secretion of amounts of alpha 1AT similar to that directed by the normal alpha 1AT complementary DNA in an in vitro eukaryotic expression system. This suggests the possibility that a human hereditary disease can be corrected by inserting an additional mutation in the same gene.

ATP-dependent fusion of liposomes with the Golgi apparatus of perforated cells

We examined the interactions of lipid vesicles (liposomes) labeled with various fluorescent markers with the intracellular membranes of semi-intact ("perforated") fibroblasts. When incubations were performed in the presence of an ATP-regenerating system, both vesicle lipids and entrapped water soluble markers were transferred to the Golgi apparatus of treated cells, indicative of membrane fusion. Fusion occurred using unilamellar vesicles 30-80 nm in diameter and composed of phosphatidylcholine alone, but was inhibited when equimolar amounts of either phosphatidylserine or sphingomyelin were present in the vesicles. Lipid vesicle-Golgi membrane fusion was also inhibited by pretreatment of the perforated cells with N-ethylmaleimide. These findings suggest that lipid vesicles may be useful for delivery of labeled lipids, macromolecules, and dyes to the Golgi apparatus, and for modeling the interactions of transport vesicles with the Golgi apparatus.