ERp44KO HeLa cells expressing Ig-λ chains were transfected with wild type (µ), L566A or C575A mutant µs chains and Halo-ERp44, or Halo-ERp44∆RDEL, as indicated. Empty vectors (- or EV) served as controls. B. The corresponding cell lysates are shown in this panel. The short black arrow points at the polymers formed in ERp44KO HeLa cells upon expression of wild type HaloERp44. The black asterisks indicate covalent complexes that this transgene forms with C575 containing µ2L2. Traces of intracellular µs2L2-ERp44∆RDEL complexes are visible in µs and L566A but absent from C575A expressing cells. Data information: All membranes were decorated with anti-μ antibodies. Black roundish spots seen in the left panels are artifacts that formed during blotting procedures and signal acquisition. 