The major intrinsic protein (MIP) from bovine lens fibre membranes has been purified from unstripped membranes using a single ion-exchange chromatography step (MonoS) in the non-ionic detergent octyl-β-D-glucopyranoside (OG). SDS-PAGE has confirmed the purity of the preparation and thin-layer chromatographic analysis has shown that the protein is virtually lipid-free. To establish a stable and monodisperse protein sample, we exchanged OG with decyl-β-D-maltopyranoside (DeM), another non-ionic detergent, by gel-filtration column chromatography. We conclude that the resulting protein/detergent complex is composed of four copies of MIP (a tetramer) and a detergent micelle. This conclusion based on: (1) measurement of the weight-average molecular mass (M̄(w,app)) of the protein moiety in the protein/detergent complex by sedimentation equilibrium; (2) measurement of the apparent molecular mass of the complexes formed by MIP in OG, in DeM, in dodecyl-β-D-maltopyranoside (DoM) and in sodium dodecylsulphate (SDS) by gel filtration; (3) measurement of the apparent molecular mass of pure detergent micelles; (4) measurement of the predicted change in the molecular mass of the MIP/DeM complex after partial enzymatic proteolysis; and (5) measurement of the size and shape of the MIP/detergent complex by electron microscopy and single-particle analysis. Therefore, the tetragonal arrangement of MIP observed in both plasma membranes and junctional membranes in lens fibre cells is maintained in solution with non-ionic detergents.
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