Identification of new Golgi complex specific proteins by direct organelle proteomic analysis

The Golgi complex is in the crossroad of the endocytic and secretory pathways. Its function is to post‐translationally modify and sort proteins and lipids, and regulate the membrane balance in the cell. To understand the structure‐function relationship of the Golgi complex the Golgi proteome has to be identified first. We have used a direct organelle proteomic analysis to identify new Golgi complex proteins. Enriched stacked Golgi membrane fractions from rat livers were isolated, and the proteins from these membranes were subsequently digested into peptides. The peptides were fractionated by cation‐exchange chromatography followed by protein identification by automated capillary‐LC/ESI–MS/MS analysis and database searches. Two different search programs, ProID and MASCOT were used. This resulted in a total of 1125 protein identifications in two experiments. In addition to the known Golgi resident proteins, a significant number of unknown proteins were identified. Some of these were further characterized in silico using different programs to provide insight into their structure, intracellular localization and biological functions. The Golgi localization of two of these newly identified proteins was also confirmed by indirect immunofluorescence.