It has become increasingly apparent that, besides targeting proteins for proteasomal degradation, ubiquitination has a role in regulating the trafficking of proteins between intracellular compartments. One well-documented example of this is the nitrogen- regulated trafficking of the general amino acid permease. Gap 1p, through the endosomal system of the yeast Saccharomyces cerevisiae. It has previously been noted that the nitrogen-regulated trafficking of Gap 1p bears similarity to the insulin-regulated trafficking of the glucose transporter, GLUT4, in fat and muscle cells. Both proteins move from an intracellular location to the cell surface in response to an extrinsic trigger. In this thesis, I show that human GLUT4 is subject to nitrogen-regulated trafficking when expressed in yeast. Furthermore, I demonstrate that, like Gap 1p trafficking in yeast, the regulated trafficking of GLUT4, both in yeast and in the insulin-sensitive cell line 3T3-L1 adipocytes, requires the ubiquitination of the transporter. A model in which ubiquitination is required for the sorting of GLUT4 into the insulin-responsive pool, from where it is mobilised to the cell surface in response to insulin, is presented and discussed.