Bluetongue virus (BTY) is an orbivirus of the Reoviridae family that infects sheep and other ruminants. BTY has three non-structural proteins, NS I, NS2 and NS3/3A. NS I forms tubular structures and its function is currently unknown. To investigate the role of NS I in BTY infection, the interactions of NS I with mammalian and insect cellular proteins, and BTY viral proteins, were examined. BTY NS I was identi tied as interacting with aldolase A, NUBP 1, Pyruvate kinase M2, cathespin B, SUM 0-1 and peptide TY7 using the yeast two-hybrid system, ELISA and immunofluorescence analysis. TY7 and NS I caused extensive cell death within 24h of co-expression; this cell death was not apoptosis and reduced BTY yield by 37%. The interaction of NS I with SUMO-I and its importance in BTY infection was confinned using siRNA to knockdown SUMO-I during BTY-IO infection. Knockdown of SUMO-I elicited a dramatic reduction in virus yield by 73%. NS I interactions with proteins of the insect vector Culicoides were also examined. A putative interaction between NS 1 and the ubiquitin activating enzyme El (UBA EI) ofCulicoides was identified during screening of a phage library, this has not been confirmed by other means. NS 1 interactions with other BTY proteins were analysed using immunoprecipitation and a strong interaction between NS 1 and YP7 was identified; this was confim1ed using the yeast two-hybrid system and immunoflourescence. Two main roles have been hypothesised for NS I from this data; firstly it is likely that NS I interaction with SUMO-I and UBA E I allows the targeting of specific proteins for sumoylation and ubiquitination allowing NS 1 to modify the host response to BTY infection. Secondly it is possible that NS I serves as an anchor for YP7 and virus cores allowing the build up of cores at the cytoskeleton in close proximity to YP2 for subsequent assembly and release. RNAi against NS J eliminated tubule formation but did not affect virus yield or YP7 and SUMO-J distribution and expression. It is therefore likely that the function of NS I does not rely on tubule fom1ation and that tubules are a form of storage for the active monomer of NSI.