The Golgi apparatus, a cellular sorting station from which proteins are shipped to various destinations, is considered an autonomous compartment with a stable, permanently localized consignment of enzymes.
But an alternative theory holds that Golgi enzymes cycle back and forth through the endoplasmic reticulum (ER), a way station that precedes the Golgi, using the same vesicular transport system as other proteins.
Now, researchers offer a fresh perspective on the former model of Golgi self-sufficiency, which was based on experiments in which Golgi enzymes were inefficiently retained in the ER in the presence of an ER trap.
The authors found that the inefficient retention is attributable to a resident FKBP13 enzyme in the ER, which causes unforeseen competitive protein binding within the ER. Experimentally hindering such competition resulted in redistribution of Golgi enzymes to the ER in the presence of the trap.
Thus, Golgi enzymes likely cycle through the ER, supporting the alternative model in which Golgi biogenesis is dependent on continual exchange with the ER. Using a modified ER trapping assay, the authors found that Golgi enzymes are shipped to the ER in tubular carriers, a process dependent on the transport-associated enzymes Rab6a and cation-independent phospholipase A2.
Further, Golgi fragmentation during microtubule disassembly and cell division likely involves enzyme recycling through the ER, according to the authors.