An essential cytoplasmic domain for the Golgi localization of coiled-coil proteins with a COOH-terminal membrane anchor

Giantin is a resident Golgi protein that has an extremely long cytoplasmic domain (about 370 kDa) and is anchored to the Golgi membrane by the COOH-te rminal membrane anchoring domain (CMD) with no luminal extension. We examin ed the essential domain of giantin required for Golgi localization by mutat ional analysis. The Gels localization of giantin was not affected by the de letion of its CMD or by substitution with the GRID of syntaxin-2, a plasma membrane protein. The giantin CMD fused to the cytoplasmic domain of syntax in-2 could not retain the chimera in the Golgi apparatus. Sequential deleti on analysis showed that the COOH-terminal sequence (positions 3059-3161) ad jacent to the CMD was the essential domain required for the Golgi localizat ion of giantin. We also examined two other Gels-resident proteins, golgin-8 4 and syntaxin-5, with a similar membrane topology as giantin. It was confi rmed that the cytoplasmic domain of about 100 residues adjacent to the CMD was required for their Golgi localization. Taken together, these results su ggest that the COOH-terminally anchored Golgi proteins with long cytoplasmi c extensions have the Golgi localization signal(s) in the cytoplasmic seque nce adjacent to the CMD. This is in contrast to previous observations that a transmembrane domain is required for Golgi localization by other Golgi pr oteins transported from the endoplasmic reticulum.