High‐yield secretion of recombinant proteins from the microalga Chlamydomonas reinhardtii
Potato virus X (PVX) has been used as an expression vector to target the green fluorescent protein (GFP) from the jellyfish Aequorea victoria to the endoplasmic reticulum (ER) of tobacco (Nicotiana clevelandii L.) leaves. Expression of free GFP resulted in strong cytoplasmic fluorescence with organelles being imaged in negative contrast. Translocation of GFP into the lumen of the ER was mediated by the use of the sporamin signal peptide. Retention of GFP in the ER was facilitated by the splicing of the ER retrieval/retention tetrapeptide, KDEL to the carboxy terminus of GFP. Fluorescence of GFP was restricted to a labile cortical network of ER tubules with occasional small lamellae and to streaming trans-vacuolar strands. Secretion of ER-targeted GFP was inhibited both by cold shock and low concentrations of the secretory inhibitor brefeldin A. However, both prolonged cold and prolonged incubation in brefeldin A resulted in the recovery of secretory capability. In leaves infected with the GFP-KDEL construct, high concentrations of brefeldin A induced the tubular network of cortical ER to transform into large lamellae or sheets which reverted to the tubular network on removal of the drug.