The Golgin GMAP210/TRIP11 Anchors IFT20 to the Golgi Complex

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Abstract
Eukaryotic cells often use proteins localized to the ciliary membrane to monitor the extracellular environment. The mechanism by which proteins are sorted, specifically to this subdomain of the plasma membrane, is almost completely unknown. Previously, we showed that the IFT20 subunit of the intraflagellar transport particle is localized to the Golgi complex, in addition to the cilium and centrosome, and hypothesized that the Golgi pool of IFT20 plays a role in sorting proteins to the ciliary membrane. Here, we show that IFT20 is anchored to the Golgi complex by the golgin protein GMAP210/Trip11. Mice lacking GMAP210 die at birth with a pleiotropic phenotype that includes growth restriction, ventricular septal defects of the heart, omphalocele, and lung hypoplasia. Cells lacking GMAP210 have normal Golgi structure, but IFT20 is no longer localized to this organelle. GMAP210 is not absolutely required for ciliary assembly, but cilia on GMAP210 mutant cells are shorter than normal and have reduced amounts of the membrane protein polycystin-2 localized to them. This work suggests that GMAP210 and IFT20 function together at the Golgi in the sorting or transport of proteins destined for the ciliary membrane. The primary cilium is a sensory organelle used by cells to monitor the extracellular environment. In mouse, severe defects in primary cilia lead to embryonic lethality while less severe defects cause a pleiotrophic phenotype that includes cystic kidney disease, retinal degeneration, obesity, and hydrocephaly, among others. The sensory functions of cilia rely on proteins localized to the ciliary membrane, which is continuous with the plasma membrane of the cell. Cells have the ability to specifically localize proteins to the ciliary membrane to the exclusion of the rest of the plasma membrane. Little is known about how this is accomplished. In prior work, we showed that the ciliary assembly protein IFT20 is localized to the Golgi complex, in addition to the cilium, and we proposed that it is involved in sorting or transport of membrane proteins to the cilium. In this work, we show that IFT20 is anchored to the Golgi complex by the golgin GMAP210. Mice defective in GMAP210 die at birth with lung and heart defects. Cells from these animals have ciliary defects, suggesting that IFT20 and GMAP210 function together at the Golgi complex in the trafficking of ciliary membrane proteins.