Inositol 1,4,5-trisphosphate receptor movement is restricted by addition of elevated levels of O-linked sugar

The inositol 1,4,5-trisphosphate receptor (InsP3R) is a versatile, ubiquitous intracellular calcium channel. Traditionally, visualizing the InsP3R in live cells involves attaching a fluorescent marker to either terminal of the protein, but the termini themselves contain binding sites for accessory molecules and proteins. Using random transposition, constructs have been developed that express the type I InsP3R with green fluorescent protein (GFP) inserted at various points within its sequence. We have used two of these constructs, one in the ligand-binding domain, and another in the regulatory domain, to investigate InsP3R dynamics within the endoplasmic reticulum. We present evidence that endogenous calcium signaling is maintained when these constructs are expressed. In addition, by measuring the fluorescent recovery after photobleaching of a subcellular region, we demonstrate that treatment with 8 mM N-acetylglucosamine (GlcNAc), known to lead to increased O-linked GlcNAcylation of proteins, leads to a reduction in the ability of the InsP3R to travel laterally within the endoplasmic reticulum. Each construct serves as the control for the other one, suggesting that this decrease in mobility is not specific to the insertion site of GFP within the InsP3R. These constructs represent a new tool that will allow us to follow receptor turnover and translocation events.