Binding to membrane proteins within the endoplasmic reticulum cannot explain the retention of the glucose-regulated protein GRP78 in Xenopus oocytes.
We have studied the compartmentation and movement of the rat 78-kd glucose-regulated protein (GRP78) and other secretory and membrane proteins in Xenopus oocytes. Full length GRP78, normally found in the lumen of rat endoplasmic reticulum (ER), is localized to a membraneous compartment in oocytes and is not secreted. A truncated GRP78 lacking the C-terminal (KDEL) ER retention signal is secreted, although at a slow rate. When the synthesis of radioactive GRP78 is confined to a polar (animal or vegetal) region of the oocyte and the subsequent movement across the oocyte monitored, we find that both full-length and truncated GRP78 move at similar rates and only slightly slower than a secretory protein, chick ovalbumin. In contrast, a plasma membrane protein (influenza haemagglutinin) and two ER membrane proteins (rotavirus VP10 and a mutant haemagglutinin) remained confined to their site of synthesis. We conclude that the retention of GRP78 in the ER is not due to its tight binding to a membrane-bound receptor.
Ceriotti
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Department of Biological Sciences, University of Warwick, Coventry, UK.
Colman
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D000818:Animals; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D018840:HSP70 Heat-Shock Proteins; D019267:Hemagglutinin Glycoproteins, Influenza Virus; D006389:Hemagglutinins, Viral / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism*; D009865:Oocytes / Q000378:metabolism*; D011499:Protein Processing, Post-Translational; D012333:RNA, Messenger / Q000378:metabolism; D013997:Time Factors; D014982:Xenopus laevis
D018840:HSP70 Heat-Shock Proteins; D019267:Hemagglutinin Glycoproteins, Influenza Virus; D006389:Hemagglutinins, Viral; D008565:Membrane Proteins; D012333:RNA, Messenger; C018551:glucose-regulated proteins