A missense mutation in the vacuolar protein GOLD36 causes organizational defects in the ER and aberrant protein trafficking in the plant secretory pathway.

A central question in cell biology is how the identity of organelles is established and maintained. Here, we report on GOLD36, an EMS mutant identified through a screen for partial displacement of the Golgi marker, ST-GFP, to other organelles. GOLD36 showed partial distribution of ST-GFP into a modified endoplasmic reticulum (ER) network, which formed bulges and large skein-like structures entangling Golgi stacks. GOLD36 showed defects in ER protein export as evidenced by our observations that, besides the partial retention of Golgi markers in the ER, the trafficking of a soluble bulk-flow marker to the cell surface was also compromised. Using a combination of classical mapping and next-generation DNA sequencing approaches, we linked the mutant phenotype to a missense mutation of a proline residue in position 80 to a leucine residue in a small endomembrane protein encoded by the gold36 locus (At1g54030). Subcellular localization analyses indicated that GOLD36 is a vacuolar protein and that its mutated form is retained in the ER. Interestingly also, a gold36 knock-out mutant mirrored the GOLD36 subcellular phenotype. These data indicate that GOLD36 is a protein destined to post-ER compartments and suggest that its export from the ER is a requirement to ensure steady-state maintenance of the organelle's organization and functional activity in relation to other secretory compartments. We speculate that GOLD36 may be a factor that is necessary for ER integrity because of its ability to limit deleterious effects of other secretory proteins on the ER.