Organization and Dynamics of the Aspergillus nidulans Golgi during Apical Extension and Mitosis

Aspergillus nidulans hyphae grow exclusively by apical extension. Golgi equivalents (GEs) labeled with mRFP-tagged PHOSBP domain form a markedly polarized, dynamic network of ring-shaped and fenestrated cisternae that remains intact during closed mitosis. mRFP-PHOSBP GEs advance associated with the growing apex where secretion predominates but do not undergo long-distance movement toward the tip that could account for their polarization. mRFP-PHOSBP GEs overlap with the trans-Golgi resident Sec7 but do not colocalize with also polarized accretions of the early Golgi marker GrhA(Grh1)-GFP, indicating that early and late Golgi membranes segregate spatially. AnSec23-GFP ER exit sites (ERES) are numerous, relatively static foci localizing across the entire cell. However, their density is greatest near the tip, correlating with predominance of early and trans-Golgi elements in this region. Whereas GrhA-GFP structures and ERES reach the apical dome, mRFP-PHOSBP GEs are excluded from this region, which contains the endosome dynein loading zone. After latrunculin-mediated F-actin disruption, mRFP-PHOSBP GEs fragment and, like AnSec23-GFP ERES, depolarize. Brefeldin A transiently collapses late and early GEs into distinct aggregates containing Sec7/mRFP-PHOSBP and GrhA-GFP, respectively, temporarily arresting apical extension. Rapid growth reinitiates after washout, correlating with reacquisition of the normal Golgi organization that, we conclude, is required for apical extension.