Retinoschisin (RS1) Interacts with Negatively Charged Lipid Bilayers in the Presence of Ca2+: An Atomic Force Microscopy Study

Abstract

Retinoschisin (RS1) is a retina-specific secreted protein encoding a conserved discoidin domain sequence. As an adhesion molecule, RS1 preserves the retinal cell architecture and promotes visual signal transduction. In young males, loss-of-function mutations in the X-linked retinoschisis gene (RS1) cause X-linked retinoschisis, a form of progressive blindness. Neither the structure of RS1 nor the nature of its anchoring and organization on the plasma membranes is fully understood. The discoidin C2 domains of coagulation factors V and VIII are known to interact with extracellular phosphatidylserine (PS). In this study we have used atomic force microscopy (AFM) to study the interactions of murine retinoschisin (Rs1) with supported anionic lipid bilayers in the presence of Ca(2+). The bilayers consisting of a single lipid, PS, and mixtures of lipids with or without PS were used. Consistent with previous X-ray diffraction studies, AFM imaging showed two distinct domains in pure PS bilayers when Ca(2+) was present. Upon Rs1 adsorption, these PS and PS-containing mixed bilayers underwent fast and extensive reorganization. Protein localization was ascertained by immunolabeling. AFM imaging showed the Rs1 antibody bound exclusively to the calcium-rich ordered phase of the bilayers pointing to the sequestration of Rs1 within those domains. This was further supported by the increased mechanical strength of these domains after Rs1 binding. Besides, changes in bilayer thickness suggested that Rs1 was partially embedded into the bilayer. These findings support a model whereby the Rs1 protein binds to PS in the retinal cell plasma membranes in a calcium-dependent manner.