Improperly folded green fluorescent protein is secreted via a non-classical pathway

The green fluorescent protein is a cytosolic protein frequently used as a molecular tag to study protein localization in intact cells. We discovered that this protein is secreted into the medium by several but not all cell lines through a non-classical secretory pathway that is insensitive to brefeldin A. Green fluorescent protein is secreted efficiently by Chinese hamster ovary cells, with 60% of synthesized proteins secreted over 8 hours. This pathway is sensitive to changes in temperature but not to factors in serum or chemicals known to affect other non-classical protein secretion pathways. Fluorescence is observed in cells expressing green fluorescent protein, indicating that some of the protein must be fully folded in the cytosol. However, secreted green fluorescent protein is not fluorescent and therefore not folded properly. Furthermore, cellular fluorescence does not change over 6 hours whereas a significant proportion of green fluorescent protein is secreted. Thus, nascent green fluorescent protein either is folded correctly or incorrectly, and the improperly folded molecules can be exported. Non-classical secretion might be a route by which cells remove an excess of improperly folded, cytosolic proteins.