D. Schematic view of the organization of µ2L2 subunits secreted by L566A and C575A mutants. For simplicity, only the tailpieces and the last constant domain (Cµ4) are shown. The L566A mutant has a disulfide bond that links two C575 in the same subunit. The open conformation of the tailpieces in C575A likely causes its slow electrophoretic mobility. The N563-glycans are processed in both mutants 