A Glycoprotein from Rat Liver Endoplasmic Reticulum Promotes Both Aggregation and Fusion of Liposomes at Acidic pH |
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Authors: | L Corazzi M Monni M Placidi R Roberti |
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Institution: | (1) Istituto di Biochimica e Chimica Medica, Università di Perugia, via del Giochetto, 06100 Perugia, Italy, IT;(2) Dipartimento di Scienze Ambientali, Università della Tuscia, via S. Camillo de Lellis, 01100 Viterbo, Italy, IT |
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Abstract: | Low-pH-induced fusion of liposomes with rat liver endoplasmic reticulum was evidenced. Fusion was inactivated by treatment
of microsomes with trypsin or EEDQ (N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline), indicating the involvement of a protein.
The protein was purified 555-fold by chromatographic steps. The identification and purification to homogeneity was obtained
by electroelution from a slab gel, which gave a still active protein of about 50 kDa. The protein promoted the fusion of liposomes;
laser light scattering showed an increase of mean radius of vesicles from 60 up to about 340 nm. Fusion was studied as mass
action kinetics, describing the overall fusion as a two-step sequence of a second order aggregation followed by a first order
fusion of liposomes. For phosphatidylcholine containing liposomes aggregation was not rate-limiting at pH 5.0 and fusion followed
first order kinetics with a rate constant of 13 · 10−3 sec−1. For phosphatidylethanolamine/phosphatidic acid liposomes aggregation was rate-limiting; however, the overall fusion was
first order process, suggesting that fusogenic protein influences both aggregation and fusion of liposomes. The protein binds
to the lipid bilayer of liposomes, independently of pH, probably by a hydrophobic segment. Exposed carboxylic groups might
be able to trigger pH-dependent aggregation and fusion. It is proposed that the protein inserted in the lipid bilayer bridges
with an adjacent liposome forming a fused doublet. Since at endoplasmic reticulum level proton pumps are operating to generate
a low-pH environment, the membrane bound fusogenic protein may be responsible for both aggregation and fusion of neighboring
membranes and therefore could operate in the exchange of lipidic material between intracellular membranes.
Received: 25 August 1997/Revised: 28 April 1998 |
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Keywords: | : Endoplasmic reticulum and fusogenic activity — Fusogenic protein — glycoprotein — Liver — Liposomes |
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