Studies on the secretion and membrane expression of IgG in mouse plasmacytoma cell lines
Most secreted proteins are hydrophilic in character, so the question arises of how a cell exports a hydrophilic secreted protein, (e.g. Immunoglobulin), through the hydrophobic lipid bilayer of the cell membrane. Two approaches to the problem were undertaken in this study. Firstly, to generate and analyse mutants which were defective in some aspect of secreted or membrane bound IgG production, and secondly, to identify and characterise membrane bound IgG to determine what differences in the membrane bound molecule directed it to the membrane and not to secretion. A number of variants of the IgG producing cell line X63.Ag8 were isolated and tested for secreted or membrane bound IgG. Of 698 individual clones tested, 55 were found to be variant in some aspect of secreted or membrane bound IgG expression. These mutant cell lines were analysed, and an attempt was made to explain the findings within the context of the data obtained. Work done on the identification and characterisation of the membrane IgG revealed that the molecule was assymetric with respect to the Ig heavy chain. The molecule was anchored in the membrane by a hydrophobic extension at the C-terminal end of one heavy chain. The other heavy chain was of the same type as is found in the secreted IgG. Inhibition of glycosylation of the membrane bound IgG had no adverse effect on its expression at the cell surface. An attempt was made to rationalise all of these data within the confines of current theories concerning the mechanisms of protein secretion.