B. Schematic representation of an IgM 'monomer'. C337 links two μ chains within a µs2L2 subunit. The conserved high mannose N-glycans which become inaccessible to Golgi enzymes in polymers are depicted as blue squares, whilst the hydrophobic stretch is in red. The aminoacidic sequence of the murine μstp is shown in the lower panel. C575 is highlighted in purple and the hydrophobic residue L566 in red. 