In the eye lens, the Major Intrinsic Protein (MIP) forms communicating channels between fiber cells appears to function in intercellular adhesion. As the eye lens ages, proteolytic activity removes the amino and carboxy termini of MIP, resulting in an alteration of its channel properties. This proteolytic cleavage also correlates with the development of lens cataracts leading to blindness. The proposed research will determine the three-dimensional structure of MIP to at least 8 angstroms resolution by electron cryo-crystallography. This method has been used to determine structures for a number of membrane transport and channel proteins, several to atomic resolution. MIP has been harvested from bovine lens tissue and purified by cation- exchange chromatography. Two-dimensional crystals of MIP will be grown by standard membrane protein crystallization techniques and recorded by cryo-electron microscopy. Image processing by back-Fourier transformation of digitized crystal records will be used to determine the channel protein structure. The primary goals of this project am to solve the structures of full- length and of proteolytically processed MIP. Secondarily, the structure of a MIP-calmodulin regulatory complex will be determined. This structural information will explain MIP function in inter-fiber cell communication and the role of MIP in lens cataract formation.
