Identification and characterization of a novel Golgi protein, GCP60, that interacts with the integral membrane protein giantin

We demonstrated previously that the integral membrane protein giantin has t he Golgi localization signal at the COOH-terminal cytoplasmic domain (Misum i, Y., Sohda, M., Tashiro, A., Sato, H., and Ikehara, Y. (2001) J Biol. Che m. 276, 6867-6873). In the present study, using this domain as bait in the yeast two-hybrid screening system, we identified a novel protein interactin g with giantin. The 3.6-kilobase mRNA encoding a 528-amino acid protein of 60 kDa designated GCP60 was ubiquitously expressed and was especially abund ant in the testis and ovary. Immunofluorescence and immunoelectron microsco py confirmed that GCP60 was co-localized with giantin in the Golgi complex. GCP60 was found to be a peripheral protein associated with the Golgi membr ane, where a COOH-terminal domain of GCP60 interacts with the COOH-terminal cytoplasmic domain of giantin. Overexpression of the COON-terminal domain of GCP60 caused disassembly of the Golgi structure and blocked protein tran sport from the endoplasmic reticulum to the Golgi. Taken together, these re sults suggest that GCP60 is involved in the maintenance of the Golgi struct ure by interacting with giantin, affecting protein transport between the en doplasmic reticulum and the Golgi.