Allen Taylor

Director, Laboratory for Nutrition and Vision Research

Professor of Nutrition, Development, Molecular & Chemical Biology, and Opthalmology

Tufts University

"From CCNY to counterintuitive regulation of protein quality: insights into eye lens development"

Abstract   

           The eye lens presents a unique opportunity to explore roles for specific molecules in cell proliferation, differentiation and development because lens cells remain in place throughout life and they go through the most extreme differentiation. Health of the eye lens is also of interest because >80% of the elderly will get cataracts or protein precipitates that obscure vision, and that can be blinding unless removed. 

           It has been >100 years since the discovery that to establish a clear lens, lens fiber cells remove their nuclei, and cease protein synthesis. But, the mechanism of this denucleation process has remained an enigma. Ubiquitination controls many critical cellular processes, most of which require specific lysines on ubiquitin (Ub). Ubiquitins are attached to substrates by the concerted action of macromolecular complexes consisting of E1, E2s, and E3s. In mammals there are few E1s, two dozen of E2s, and hundreds of E3s. Ubiquitin has 7 lysines (K).  For most substrates, attachment of K48-linked ubiquitin chains assures their degradation. <5% of conjugates employ K6 on ubiquitin and least is known about effects of modification of K6. Expression of K6W-ubiquitin in the lens and lens cells results in accumulation of intracellular aggregates and also slows cell proliferation and the differentiation program, including expression of lens specific proteins, differentiation of epithelial cells into fibers, achieving proper fiber cell morphology, and removal of cell nuclei. We found that denucleation is controlled by Cdk1 in cooperation with p27. Cell cycle is also driven in part by p27. Accumulation of po27 is a common characteristic of most models of cataract. Curiously, p27  is stabilized, rather than destabilized by ubiquitin conjugating enzyme 7, Ubc7 (Ube2L3). This appears to be a consequence of assembling stabilizing rather than destabilizing ubiquitin conjugates. Expression of K6W-Ub also causes hyperactivation of calpain.  What is the E3 that completes ubiquitination of p27? Hopefully, I’ll answer that question.