Reconstitution of Functional Integrin into Phospholipid Vesicles and Planar Lipid Bilayers - Methods in Molecular Biology: Extracellular Matrix Protocols - page 67

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Reconstitution of Functional Integrin
into Phospholipid Vesicles
and Planar Lipid Bilayers

Eva-Maria Erb and Jürgen Engel

1. Introduction
Integrins are heterodimeric cell-surface receptors involved in a variety of
functions such as binding to the ECM, regulation of the cellular organization,
cell migration, proliferation, differentiation, and gene expression (1 , 2) .
Integrins in their native environment can diffuse in the plane of the membrane,
and this mobility is required for processes like the assembly of integrins to
focal contacts or cell migration (3 , 4) .
The integrin aIIIbb3 was used in the present study as a model integrin
because of the relative ease of isolation. It is the main receptor on blood plate-
lets and plays an important role in the coagulation process. The properties of
the purified receptor, e.g., ligand binding or receptor mobility, were investi-
gated using integrin 3 reconstituted into phospholipid vesicles and

supported planar bilayers. As compared to studies with whole cells, both
systems allow an investigation of the receptor without the complex influences
of cytoplasmic components and other membrane proteins, but with the recep-
tor still embedded in a lipid membrane. An advantage of the bilayer system
compared to solid-phase assays is the receptors' mobility and native state. A
thin water layer of about 30 Å between the bilayer and the support (5) is
important for the lateral mobility of lipids and proteins in the plane of the mem-
brane. Proteins adsorbed on lipid coated surfaces are often highly restricted in
their lateral motion (6) and when directly adsorbed on plastic, proteins are
immobilized and the native state may be lost by adsorption (7) . Furthermore,
the surface concentration of the receptor may be controlled in the vesicle and
bilayer system.

α IIb β