Novel protein-protein interactions in the lens: a solution to the Mp20 enigma

Abstract

The lens is unique in that it packs its cells in a tight crystalline order to maintain tissue transparency. To do that, it evolved into a highly specialized organ, developing a smart microcirculation system to provide nutrients to deeper lying cells, and an arsenal of membrane proteins that act as adhesion facilitators to bring together apposing cells. The second most abundant lens fiber cell membrane protein is Mp20 which was localized to junctional and single membrane (non-junctional) areas. It has no known function, and binding partners, an unresolved membrane topology and structure. It is an enigmatic protein. My thesis provides new data that contribute to a better understanding of lens MP20. MP20 was purified and characterized biochemically and structurally. Membrane proteomics discovered a new adhesion protein in the lens, galectin-3, which turned out to be a ligand for MP20. An MP20: galectin-3 complex was isolated from lens fiber cells. A model shows how the two binding partners could zip up adjacent cell membranes thereby minimizing light scattering and thus supporting tissue transparency. The structure of MP20 was analysed by single particle reconstruction and electron crystallography of ordered two-dimensional arrays after reconstitution into artificial bilayers. A projection map of MP20 to 18Å and three-dimensional models of various MP20 complexes constitute the first visualization of this enigmatic protein. Overall, my results strengthen the view that MP20 serves an adhesion role. However, some aspects of the structure hint at the possibility that MP20 might also serve as a channel. Such potential function for MP20 would parallel the recently established dual role of the most predominant lens membrane protein, MIP, which is both an aquaporin as well as an adhesion protein. Focussed electrophysiology experiments need to be done with MP20 to establish a possible channel activity. If confirmed, evolution of the lens would have indeed made very efficient use of its major membrane proteins!