Asymmetrical surface IgG on MOPC-21 plasmacytoma cells contains membrane heavy chain and one secretory heavy chain

Membrane proteins from the B lymphomas WEHI-231 and 2PK3 and from the plasmacytomas MPC-11 and MOPC-21 were radioiodinated in situ by the lactoperoxidase method and were subjected to two-dimensional (nonreduced, reduced) polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Few heavily labeled membrane proteins were composed of disulfide-bonded subunits. One such protein (m.w. 200,000 intact and 116,000 reduced) shared some properties with the PC-1 alloantigen, although it was not conclusively identified. A second major disulfide-bonded protein (m.w. 200,000 intact and 95,000 reduced) has been identified previously as the receptor for transferrin. Membrane immunoglobulins of WEHI-231 (IgM) and 2PK3 (IgG2a) had the expected subunit structure, whereas membrane immunoglobulin was not detected on MPC-11. In contrast, surface IgG1 of MOPC-21 appeared to consist almost entirely of hybrid molecules containing one membrane gamma 1 chain and one secretory gamma 1 chain. This hybrid IgG molecule appeared to exist in both monomeric and dimeric forms. It is concluded that i) the synthetic and assembly mechanisms of secretory and membrane IgG1 are shared; ii) there are no special mechanisms to prevent pairing of membrane and secretory gamma 1 chains; iii) the presence of one hydrophobic tail is sufficient for membrane insertion of gamma 1 chains; and iv) the C-terminal extension cysteine residues of membrane gamma 1 chains in hybrid IgG molecules are either unpaired or may allow the formation of hybrid IgG dimers.