Structural evidence for multiple transport mechanisms through the Golgi inthe pancreatic beta-cell line, HIT-T15

Accurate data on the three-dimensional architecture of the Golgi is prerequ isite for evaluating the mechanisms of transit through this organelle. Here we detail the structure of the Golgi ribbon within part of an insulin-secr eting cell in three dimensions at similar to6 nm resolution. Rapid freezing , freeze-substitution and electron tomography were employed. The Golgi in t his region is composed of seven cisternae. The cis-most element is structur ally intermediate between the endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC) and the cis-most cisterna characterized in three dimen sions at high resolution in a normal rat kidney cell [Ladinsky, Mastronarde , McIntosh, Howell and Staehelin (1999) J. Cell Biol. 144, 1135-1149]. Ther e are three transcisternae that demonstrate morphological and functional va riation. The membrane surface areas and volumes of these elements decrease from cis to trans. The two trans-most cisternae are dissociated from the st ack and are fragmented by tubulation. ER closely adheres to and inserts bet ween individual trans-cisternae. Many of the 2119 small, clathrin-negative vesicles that are in close proximity to the Golgi fill the region where tra nscisternae have moved out of register with the ribbon. These data provide evidence that cisternal progression/maturation, trafficking via membrane tu bules and vesicle-mediated transport act in concert in the same region of t he Golgi ribbon, and suggest an important role for the ER in regulating mem brane dynamics at the trans-Golgi.