The Golgi apparatus is specialized for glycosylation reactions. In this organelle, many plant cell wall polysaccharides and proteoglycans are synthesized. To discover proteins involved in the synthesis of these glycans, we are studying the protein composition of the Golgi apparatus. Localization of Organelle Proteins by Isotope Tagging (LOPIT) is a high-throughput technique for localization of proteins by mass spectrometry (Dunkley et al. 2004; Dunkley et al. 2005). Using LOPIT we have identified nearly 100 membrane proteins in the Golgi apparatus of Arabidopsis thaliana. These Golgi proteins included previously studied enzymes of nucleotide sugar metabolism such as UXS4 and GAE6. There were several previously described glycosyltransferases including QUA1 and others including GT8, GT34 and GT47. Bioinformatic analyses of the uncharacterized proteins suggest that they include putative novel glycosyltransferases, transporters, and a number of putative seven transmembrane domain (TMD)-containing proteins. A substantial proportion of the Golgi proteins show homology to methyltransferases. We hypothesize that many of the novel proteins are involved in synthesis of cell wall polysaccharides. We present here an analysis of the proteins found in the Golgi apparatus of Arabidopsis callus.