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 8-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

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... chanisms 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.