Conformational properties of the main intrinsic polypeptide (MIP26) isolated from lens plasma membranes

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Abstract
The conformational properties of the main intrinsic polypeptide (MIP26) isolated from lens plasma membranes were studied by using near- and far-ultraviolet circular dichroism. The far-ultraviolet spectrum of MIP26 solubilized with octyl .beta.-D-glucopyranoside indicates an .alpha.-helical content of .apprx. 50% and a .beta.-structure content of .apprx. 20%. A detergent-free membrane suspension of MIP26 produced a typically distorted far-ultraviolet spectrum which was caused by differential light scattering and absorption flattening. However, decreasing the size of the membrane fragments by sonication produced a far-ultraviolent spectrum free of distortion, and with a rotatory strength profile similar to that obtained for MIP26 solubilized with octyl .beta.-D-glucopyranoside. This implies similar secondary structure properties for protein in both the suspension and the sugar detergent. The cleavage of MIP26 with Staphylococcus aureus protease, which results in removal of a 5-kilodalton peptide and which mimics the age-dependent posttranslational changes that take place in the lens, did not significantly affect the conformation of the core protein as judged by the near-ultraviolet circular dichroism spectra.

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