Electron T omography R eveals R ab6 I s E ssential to the T rafficking of trans ‐ G olgi C lathrin and COPI ‐ C oated V esicles and the M aintenance of G olgi C isternal N umber

We have shown previously that R ab6, a small, trans‐ G olgi‐localized GTP ase, acts upstream of the conserved oligomeric G olgi complex ( COG ) and ZW 10/ RINT 1 retrograde tether complexes to maintain G olgi homeostasis . I n this article, we present evidence from the unbiased and high‐resolution approach of electron microscopy and electron tomography that R ab6 is essential to the trans‐ G olgi trafficking of two morphological classes of coated vesicles; the larger corresponds to clathrin‐coated vesicles and the smaller to coat protein I ( COPI )‐coated vesicles . On the basis of the site of coated vesicle accumulation, cisternal dilation and the normal kinetics of cargo transport from the endoplasmic reticulum ( ER ) to G olgi followed by delayed G olgi to cell surface transport, we suggest that G olgi function in cargo transport is preferentially inhibited at the trans ‐ G olgi/ trans ‐ G olgi network ( TGN ) . The >50% increase in G olgi cisternae number in R ab6‐depleted H e L a cells that we observed may well be coupled to the trans ‐ G olgi accumulation of COPI ‐coated vesicles; depletion of the individual R ab6 effector, myosin IIA , produced an accumulation of uncoated vesicles with if anything a decrease in cisternal number . These results are the first evidence for a R ab6‐dependent protein machine affecting G olgi‐proximal, coated vesicle accumulation and probably transport at the trans ‐ G olgi and the first example of concomitant cisternal proliferation and increased G olgi stack organization under inhibited transport conditions .