The RER-Localized Rotavirus Intracellular Receptor: A Truncated Purified Soluble Form Is Multivalent and Binds Virus Particles

A budding event transfers the immature, single-shelled rotavirus particle (SSP) across the RER membrane prior to assembly of mature virions in the ER lumen. Budding is triggered by the interaction of the SSP with a viral receptor glycoprotein (NS28) which is located in the RER membrane. We have expressed the cytoplasmic domain of the NS28 receptor as a glutathione S-transferase fusion protein to generate a soluble polypeptide that in turn can be cleaved to yield a carboxy-terminal receptor domain. The soluble terminal domain (Δ1-85 NS28) has been purified to homogeneity and retains SSP-binding activity when immobilized on a solid matrix. Integral membrane status therefore is not an essential prerequisite for ligand binding. The K d for the interaction between immobilized Δ1-85 NS28 and purified particles is 4.6 × 10 -11 M, a value indistinguishable from the value obtained for the full-length and membrane-anchored receptor. Cross-linking with the bifunctional reagent dimethylsuberimidate indicates that Δ1-85 NS28 is a tetramer. When Δ1-85 NS28 is added to a monodisperse suspension of purified virus, the particles aggregate, indicating that the receptor is multivalent. The rotavirus intracellular receptor therefore provides a model for the detailed analysis of the early events that trigger the budding of cytoplasmically located particles across cell membranes.