Retinoschisin at 4 Å Resolution from cryo-EM: A Junctional Model of Back-to-Back Octamers for Adhesion in the Retina
The protein retinoschisin (RS1) plays an important role in cell-cell adhesion in the retina. Mutations in RS1 lead to X-linked retinoschisis (XLRS), a form of vision impairment in young males characterized by separation of inner retinal layers, among other effects. We expressed and purified RS1 and determined its structure by cryo-electron microscopy at ∼ 4 Å, assisted by data from a direct detection camera. We visualize octameric rings, paired back-to-back and in register. RS1 (24 kDa) has a small N-terminal domain of unknown structure and a discoidin domain, for which crystal structures of homologs are known. The RS1 domains occupy the centers of the rings but are less clearly defined, suggesting mobility. We refined the cryo-EM structure of the discoidin rings by flexible fitting, and requiring consistency with known intramolecular and intermolecular disulfides. In the resulting model, the interfaces between subunits in a ring and between rings involve residues to which XLRS mutations have been mapped. We propose that RS1 couples adjacent cells through octamer-octamer contacts. Homotypic interactions between opposing protein assemblies are reminiscent of other kinds of junctions (e.g. gap junctions) but RS1differs in that it has no transmembrane component but must anchor to the membrane in some other way; this may perhaps involve the binding of RS1 domains to receptors.