Capsular polysaccharides are important virulence determinants in a wide range of invasive infectious diseases. Although capsule synthesis has been extensively investigated, understanding polysaccharide export from the cytoplasm to the external environment has been more difficult. Here we present the results of a novel protection assay indicating that synthesis and export of the Escherichia coli K1 group 2 capsular polysialic acid (K1 antigen) occur within a protected subcellular compartment designated the sialisome. In addition to the polymerase encoded by neuS, localization and complementation analyses indicated that the sialisome includes the accessory membrane protein NeuE. The requirement for NeuE was suppressed by overproducing NeuS, suggesting that NeuE functions by stabilizing the polymerase or facilitating its assembly in the sialisome. Although an interaction between NeuE and NeuS could not be demonstrated with a bacterial two-hybrid system that reconstitutes an intracellular cell-signalling pathway, interactions between NeuS and KpsC as well as other sialisome components were detected. The combined results provide direct evidence for specific protein–protein interactions in the synthesis and export of group 2 capsular polysaccharides under in vivo conditions. The approaches developed here will facilitate further dissection of the sialisome, suggesting similar methodology for understanding the biosynthesis of other group 2 capsules.