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Studies of asymmetric membrane assembly
Abstract:
The major capsid protein of M13 bacteriophage is incorporated at each stage of infection into the host plasma membrane with its amino terminus exposed on the outer surface. Purified M13 coat protein is incorporated with the same asymmetry into synthetic phosphatidylcholine vesicles formed near the Tm of the lipid by a cholate dilution technique. We now report that the lipid in the pre-dilution mixture exists as mixed micelles of uniform size. Prior to dilution, the coat protein is present in at least two states of aggregation, both of which behave similarly in the model membrane assembly reaction. No detectable lipid-protein interaction occurs prior to dilution. Upon dilution there is rapid production of small closed vesicles and coat protein is converted to a chymotrypsin-resistant form, presumably reflecting its incorporation into these vesicle bilayers. Formation of large (greater than 6000 A diameter) vesicles occurs slowly with preservation of coat protein asymmetry and internal volume. A model for this assembly reaction is proposed.
Insights
M13 bacteriophage coat protein incorporates into host membranes with its amino terminus outward. This study reveals how purified M13 coat protein self-assembles into lipid vesicles, forming uniform structures with preserved protein asymmetry.
Area of Science:
- Biophysics
- Molecular Biology
- Membrane Protein Assembly
Background:
- The M13 bacteriophage major capsid protein integrates into host plasma membranes during infection.
- This protein exhibits specific asymmetric insertion with its amino terminus exposed externally.
- Understanding this process is key to viral assembly and membrane protein dynamics.
Purpose of the Study:
- To investigate the self-assembly mechanism of purified M13 coat protein into synthetic lipid vesicles.
- To elucidate the role of lipid-protein interactions in vesicle formation and protein incorporation.
- To characterize the structural states of M13 coat protein during vesicle assembly.
Main Methods:
- Cholate dilution technique to form phosphatidylcholine vesicles near the lipid's phase transition temperature (Tm).
- Analysis of lipid-protein mixtures using techniques to assess micelle and vesicle formation.
- Chymotrypsin digestion to determine protein structural changes and incorporation into vesicles.
Main Results:
- Lipid in pre-dilution mixtures formed uniform-sized mixed micelles.
- M13 coat protein existed in multiple aggregation states prior to dilution, with no initial lipid interaction.
- Dilution rapidly produced small, closed vesicles with incorporated coat protein (chymotrypsin-resistant).
- Large vesicles formed slowly, maintaining coat protein asymmetry and internal volume.
Conclusions:
- M13 coat protein self-assembly into vesicles is a dilution-dependent process.
- Protein incorporation into vesicles occurs rapidly upon dilution, resulting in a stable, asymmetric structure.
- A model for M13 coat protein-lipid vesicle assembly is proposed, highlighting the importance of lipid micelle intermediates.