Allosteric regulation of lysosomal enzyme recognition by the cation-independent mannose 6-phosphate receptor
The cation-independent mannose 6-phosphate receptor (CI-MPR), also known as the IGF2 receptor or CD222, is a multifunctional type I transmembrane glycoprotein ubiquitously expressed in most eukaryotic cell types. Through the receptors ability to bind a variety of unrelated extracellular and intracellular ligands, it is involved in a wide array of functions including protein trafficking, lysosomal biogenesis, internalization, regulation of cell growth, cell migration and apoptosis. CI-MPR has a large extracellular region comprised of 15 contiguous domains, four of which interact with phosphorylated glycans on lysosomal enzymes. Here we present a series of biophysical studies, along with crystal structures, providing information on how the N-terminal 5 domains of this receptor work in concert to bind and release carbohydrates. High-resolution electron microscopy as well as hydroxyl radical protein footprinting (HRPF) of this multifunctional multidomain construct demonstrates dynamic conformational changes occur as a consequence of ligand binding and different pH conditions, These data, coupled with surface plasmon resonance studies and molecular modeling, allow us to propose a bi-dentate oligosaccharide binding model, which could explain how high affinity carbohydrate binding is achieved through allosteric domain cooperativity.