The Golgin GMAP-210

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.