Protein secretion in plant cells can occur via a default pathway

To study protein secretion in plant cells, we established and evaluated a model system based on transient synthesis of heterologous proteins in tobacco protoplasts. We show that the nonsecretory enzymes phosphinothricin acetyl transferase, neomycin phosphotransferase II, and beta-glucuronidase are secreted when targeted to the lumen of the endoplasmic reticulum by signal peptide-mediated translocation. These data are consistent with the view that secretion can occur independent of active sorting mechanisms by nonspecific migration through the exocytic pathway. However, the rate of secretion differs significantly among these enzymes. Furthermore, the presence of signal sequences was found to be correlated with a reduction of the levels of the encoded gene products. This is the result of post-transcriptional events that limit either synthesis or stability of the proteins in vivo.