Probing the structure of rotavirus NSP4: a short sequence at the extreme C terminus mediates binding to the inner capsid particle.

The rotavirus nonstructural glycoprotein NSP4 functions as the receptor for the inner capsid particle (ICP) which buds into the lumen of the endoplasmic reticulum during virus maturation. The structure of the cytoplasmic domain of NSP4 from rotavirus strain SA11 has been investigated by using limited proteolysis and mass spectrometry. Digestion with trypsin and V8 protease reveals a C-terminal protease-sensitive region that is 28 amino acids long. The minimal sequence requirements for receptor function have been defined by constructing fusions with glutathione S-transferase and assessing their ability to bind ICPs. These experiments demonstrate that 17 to 20 amino acids from the extreme C terminus are necessary and sufficient for ICP binding and that this binding is cooperative. These observations are consistent with a model for the structure of the NSP4 cytoplasmic region in which four flexible regions of 28 amino acids are presented by a protease-resistant coiled-coil tetramerization domain, with only the last approximately 20 amino acids of each peptide interacting with the surface binding sites on the ICP.