Mechanism for the Organization of the Golgi apparatus

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The Golgi apparatus consisting of stacks of cisternae is a station for the transit of secretory proteins. Membranes flow into and out from the Golgi apparatus as secretary proteins are transported from the endoplasmic reticulum to the plasma membrane through this organelle. In addition, membrane efflux occurs in accordance with retrograde transport from the Golgi to the endoplasmic reticulum. In spite of massive membrane flow, the structure of the Golgi apparatus is maintained during the interphase of mammalian cells. The purpose of the present project is to elucidate the mechanism of the organization of the Golgi apparatus, and to identify new proteins involved in vesicular transport in the early secretory pathway. The following results were obtained.1. Nordihydroguaiaretic acid-induced Golgi disassembly was prevented by overexpression of Gαi2 or Gαz, but not other Gα species. Expression of RGS (regulator of G-protein signaling) proteins specific for Gαz also caused Golgi disassembly, suggesting that the active form of Gαz plays a role in the maintenance of the Golgi apparatus.2. Short chain ceramide blocked Golgi disassembly caused by Golgi-disrupting reagents such as brefeldin A, whereas long chain ceramide enhanced their effects. This suggests that sphingolipid metabolism is implicated in the stability of the Golgi apparatus.3. A novel protein termed p125 that can interact with a subunit of COPII, Sec23p, was isolated. It possesses a Pro-rich region and a phospholipase A domain. p125 was colocalized with a tethering protein, p115. A data base search revealed the presence of a protein (KIAA0725p) which shows high similarity to p125.4. A novel syntaxin species (syntaxin 18) localized in the endoplasmic reticulum was identified. Syntaxin 18 is possibly involved in the retrograde transport. Several syntaxin 18-binding proteins were identified and their characterization is now in progress.