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Secretion of active Fc fragments of immunoglobulin E directed by the yeast invertase signal sequence in mammalian cells.

The human Fc fragments of immunoglobulin E (IgE-Fc) expressed in the mammalian COS cells, those mainly consisting of C epsilon 3-C epsilon 4 chains with or without Cys-328 known to be responsible for interchain disulfide bonding, were secreted into the culture medium directed by the yeast Saccharomyces cerevisiae invertase (SUC2) signal sequence (SUC2/Ig2 or SUC2/Ig, respectively) as well as by the human interleukin 2 receptor alpha chain signal sequence (IL2R/Ig2 or IL2R/Ig, respectively). For the binding activity to the human soluble alpha chain of high affinity receptor for IgE, IgE-Fc with Cys328, SUC2/Ig2 and IL2R/Ig2, were active but had smaller activity than native IgE. By comparison IgE-Fc without Cys328, SUC2/Ig and IL2R/Ig, showed lower activity than SUC2/Ig2 and IL2R/Ig2. Immunoprecipitation analysis revealed both SUC2/Ig and SUC2/Ig2 had molecular weights (M(r)s) and degrees of glycosylation similar to IL2R/Ig and IL2R/Ig2, respectively. These results suggested that in mammalian cells the SUC2 signal sequence was functional in directing the heterologous multimeric proteins to the endoplasmic reticulum, resulting in secretion of the active proteins. This finding might show merits on heterologous protein secretion systems.