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Egerer, Johannes (2008): The Golgin GMAP-210: Functions and Mechanisms. Dissertation, LMU München: Faculty of Biology
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Abstract

The protein GMAP-210 (Golgi Microtubule Associated Protein of 210 kDa) is a long coiled-coil protein, which localises to the Golgi apparatus. It is part of the loosely defined protein group of the golgins, which are involved in establishing the Golgi morphology and in vesicular trafficking around the Golgi. By using biochemical, cell biological and molecular biological methods GMAP-210 was examined in regards to its Golgi targeting capability, its interaction partners and its function in establishing Golgi morphology and positioning. In vitro and in vivo experiments showed that GMAP-210 targets to the Golgi via its C-terminal GRAB domain. Its proposed interaction with Arf1, however, could not be definitely determined, although there is strong evidence for it. Arf1 binding to the GRAB domain was hindered in the full-length protein, but not with short C-terminal fragments containing the minimal GRAB domain. This implies that additional factors are needed for GMAP-210 Golgi binding. A yeast 2-hybrid screen of the entire family of small Rab GTPases identified the Golgi and ER localised Rab1 as a novel interaction partner of GMAP-210. GMAP-210 also labels vesicular tubular structures in the cell, which partially overlap with COPII and ERGIC53, components of the early secretory pathway. This gives additional evidence that GMAP- 210 is involved in ER to Golgi transport. Trafficking of a model substrate, the vesicular stomatitis virus G-protein (VSV-G), however, was not impaired in the absence of GMAP- 210. This indicates that GMAP-210 functions only in specialised transport pathways. Knockdown of GMAP-210 in HeLa L cells by siRNA changed the Golgi morphology and the Golgi fragmented into a cluster of vesicles. Its overexpression caused the Golgi to grow long tubular structures. Both effects on morphology could only be observed in HeLa L cells, not in hTERT-RPE1 cells. As direct interaction with microtubules or γ-tubulin could not be detected, and GMAP-210 is therefore unlikely to affect Golgi morphology by directly perturbing microtubule function. GMAP-210 knockdown by siRNA also showed its interaction with the intraflagellar transport protein IFT20. This protein lost its Golgi localisation when GMAP-210 was depleted. Both proteins interacted directly. GMAP-210, however, was not involved in primary cilium formation in hTERT-RPE1 cells and loss of IFT20 from the Golgi did not impair formation of the cilium, proposing that the Golgi pool of IFT20 had a function apart from intraflagellar transport and formation of the primary cilium. These results set GMAP-210 apart from the archetypal golgins GM130 and p115 and indicate that GMAP-210 is involved in a highly specialised transport pathway, which could nevertheless influence the morphology of the Golgi apparatus in certain cell types.