Regulated secretion is crucial to many cellular functions and is the pathway that neurotransmitters, digestive enzymes and hormones utilise to leave the cell, often in response to a specific stimulus. Insulin secretion from pancreatic β cells is a biphasic response induced by increased extracellular glucose levels. The first rapid phase of secretion requires the opening of voltage-dependent calcium channels resulting in increased intracellular calcium concentrations. This triggers exocytosis from a pool of previously docked and primed insulin-containing granules. The second phase, defined by prolonged, gradually increasing insulin secretion, requires recruitment of granules from reserve pools and is dependent on granules being made competent for exocytosis. Uncertainty exists over what the primary sensor of increased calcium levels is and what the molecular mechanisms responsible for vesicle priming are. Understanding these mechanisms is crucial to our understanding of the secretory defect in Type II diabetes, where there is a loss of the first phase and a reduction in the second phase of glucose-stimulated insulin secretion. Tumour protein D52 (TPD52) interacts with several proteins involved in membrane trafficking, e.g. annexin VI (1) and MAL2 (2), and is involved in the regulated secretion of digestive enzymes from pancreatic acinar cells after secretagogue stimulation (3). We found expression of TPD52 in both the rat and mouse pancreatic β cell lines, INS-1 and MIN6, and hypothesise that TPD52 is involved in regulated secretion of insulin from the β cell. To determine the effect of TPD52 on regulated secretion, we have knocked down TPD52 RNA levels using a vector based siRNA approach. The siRNA constructs were checked for efficacy against an HA-tagged TPD52 transgene and then a stable knockdown cell line was created by transfecting the TPD52-siRNA vector into INS-1 cells and selecting G418 resistant colonies. Two clonal lines were created that showed a 50 % and an 80 % reduction in TPD52 RNA levels. Western blot analysis confirmed a marked reduction in TPD52 protein levels in the latter cell line. Initial data showed that this reduction in TPD52 protein levels did not affect glucose-stimulated insulin secretion over a 3 hour stimulation period. Isoelectric focusing gels, however, showed that in normal INS-1 cells TPD52 was rapidly phosphorylated into three phosphoisoforms only two minutes after glucose stimulation. Phosphorylation was transient, peaking at 15 min post-stimulation and returning to basal levels within 1 hour. This suggests that TPD52 may be involved only in the initial rapid phase of insulin secretion and work is currently underway to determine the effects of TPD52 knockdown on the first phase of glucose-stimulated insulin secretion.