| Abstract: | Proteins that enter and pass through the endomembrane system ultimately migrate to either the vacuole, or the cell surface. It is unclear however how the plant cell discriminates and directs protein trafficking between these two locations. Prior to this work, endomembrane proteins lacking specific targeting information other than a signal peptide were assumed to be secreted to the cell surface through a "default" pathway. The major conclusion of this work is that the cellular trafficking routes for unmarked (i.e. non-targeted) proteins may be much more complex than asserted by the default secretion concept Using immunolocalization techniques with electron microscopy, we show that an N-terminal amino acid signal peptide derived from the cell wall protein extensin is sufficient to mediate translocation of the bacterial protein beta-glucuronidase (GUS) across the endoplasmic reticulum and into the plant endomembrane system. GUS activity is negligible in transformed plants, but increases dramatically after treatment with tunicamycin-an N-linked glycosylation inhibitor. Secretion of GUS is unaffected by protein glycosylation and tunicamycin-induced inhibition of glycosylation. Subcellular fractionation analysis corroborated our immunolocalization results, which revealed GUS in the vacuole as well as the cell wall region of transgenic plants suggesting overlap between these two pathways. |
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