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A fusogenic protein from rat brain microsomal membranes: partial purification and reconstitution into liposomes
M Rakowska1, J Zborowski, L Corazzi
1Institute of Biochemistry and Medical Chemistry, University of Perugia, Italy.
Abstract:
The procedures for purification and reconstitution of rat brain microsomal membrane protein that causes fusion of liposomes at acidic pH are described. A 1,860-fold purification was achieved, starting from the detergent-solubilized microsomal membranes. The fusion process was assayed spectrofluorimetrically by monitoring the formation of terbium-dipicolinic acid complex (Wilschut, J. et al. 1980. Biochemistry 19:6011-6021) evoked by the protein after mixing of two populations of liposomes. The fusogenic activity of the protein inserted into the membrane of Tb3+-containing vesicles was found to be strongly dependent on phospholipid composition and was higher in vesicles enriched with exogenous phosphatidylserine, phosphatidylglycerol and phosphatidylethanolamine than in those prepared with an excess of phosphatidylcholine. The vesicles enriched in negatively charged phospholipids were bound to Concanavalin A coupled to Sepharose-4B and could be released from this column only in the presence of a high concentration of alpha-methylmannopyranoside and detergent, indicating a glycoprotein nature of the fusogenic protein. Furthermore, these data show that protein inserted into membrane has its oligosaccharide chains exposed to the environment.
Insights
Researchers purified a rat brain protein causing liposome fusion at acidic pH. Its activity depends on phospholipid composition, and it is a glycoprotein with exposed oligosaccharides.
Area of Science:
- Biochemistry
- Membrane Biology
- Protein Chemistry
Background:
- Microsomal membrane proteins play crucial roles in cellular processes.
- Liposome fusion is a key mechanism in membrane trafficking and drug delivery.
- Understanding protein-mediated membrane fusion requires isolating and characterizing fusogenic proteins.
Purpose of the Study:
- To purify and characterize a rat brain microsomal membrane protein responsible for liposome fusion at acidic pH.
- To investigate the influence of phospholipid composition on the fusogenic activity of this protein.
- To determine the structural and functional properties of the purified fusogenic protein.
Main Methods:
- Purification of rat brain microsomal membrane protein using detergent solubilization.
- Assay of liposome fusion using spectrofluorimetry to monitor terbium-dipicolinic acid complex formation.
- Analysis of protein-phospholipid interactions and binding to Concanavalin A-Sepharose-4B.
Main Results:
- Achieved a 1,860-fold purification of the fusogenic protein.
- Fusogenic activity was significantly higher in liposomes enriched with negatively charged phospholipids (phosphatidylserine, phosphatidylglycerol, phosphatidylethanolamine) compared to phosphatidylcholine.
- The protein demonstrated binding to Concanavalin A, indicating its glycoprotein nature, with oligosaccharide chains exposed externally.
Conclusions:
- A novel fusogenic glycoprotein from rat brain microsomes was purified and characterized.
- Phospholipid composition critically influences the fusogenic activity of this protein.
- The exposed oligosaccharide chains suggest specific interactions or roles in membrane fusion events.