A mis-sense point mutation in the human VAPB gene is associated with a familial form of motor neuron disease that has been classified as Amyotrophic Lateral Sclerosis type VIII. Affected individuals suffer from a spinal muscular atrophy (SMA), amyotrophic lateral sclerosis (ALS) or an atypical slowly progressing form of ALS. Mammals have two homologous VAP genes, vapA and vapB. VAPA and VAPB share 76% similar or identical amino acid residues; both are COOHterminally anchored membrane proteins enriched on the endoplasmic reticulum. Several functions have been ascribed to VAP proteins including membrane trafficking, cytoskeleton association and membrane docking interactions for cytoplasmic factors. It is shown here that VAPA and VAPB are expressed in tissues throughout the body but at different levels, and that they are present in overlapping but distinct regions of the endoplasmic reticulum. The disease-associated mutation in VAPB, VAPB (P56S) is within a highly conserved N-terminal region of the protein that shares extensive structural homology with the major sperm protein (MSP) from nematodes. The MSP domain of VAPA and VAPB is found to interact with the ERlocalized transcription factor ATF6. Over expression of VAPB or VAPB (P56S) attenuates the activity of ATF6-regulated transcription and the mutant protein VAPB (P56S) appears to be a more potent inhibitor of ATF6 activity. Moreover VAP proteins affect the activity of XBP1 and BiP promoter elements, two major components of the Unfolded Protein Response (UPR) of the Endoplasmic Reticulum and the different domains of VAPB have a differential effect on UPR regulation. Finally, over expression of the MSP domain of VAPB leads to cell death via apoptosis, while overexpression of other VAPB domains renders cells more susceptible to apoptotic death after ER stress. The data presented in this thesis indicate that VAP proteins interact directly with components of ER homeostatic and stress signalling systems and may therefore be parts of a previously unidentified regulatory pathway. The mis-function of such regulatory systems may contribute to the pathological mechanisms of degenerative motor neuron disease.