Further, 3b was found to increase the transactivation potential of RUNX1b on the IL2 promoter, which may partly be attributed to the enhanced ERK dependent phosphorylation of RUNX1b in 3b expressing cells. and co-immunoprecipitation techniques, we have identified a host transcription factor RUNX1b (Runt related transcription factor, isoform b) as a novel interacting partner for SARS-CoV 3b protein. Chromatin immunoprecipitaion (ChIP) and reporter gene assays in 3b expressing jurkat cells showed recruitment of 3b on the RUNX1 binding element that led to an increase in RUNX1b transactivation potential on the IL2 promoter. Kinase assay and pharmacological inhibitor treatment implied that 3b also affect RUNX1b transcriptional activity by regulating its ERK dependent phosphorylation levels. Additionally, mRNA levels of MIP-1, a RUNX1b target gene upregulated in SARS-CoV infected monocyte-derived dendritic cells, were found to be elevated in 3b expressing U937 monocyte cells. Conclusions/Significance These results unveil a novel interaction of SARS-CoV 3b with the host factor, RUNX1b, and speculate its physiological relevance in upregulating cytokines and chemokine levels in state of SARS virus infection. Introduction Severe acute respiratory syndrome (SARS) emerged in the Guangdong province of China in November 2002 and swept through more than 29 countries. Its spread infected more than 8000 people with a high mortality rate of 10%. It was found to be associated with a novel coronavirus named SARS-CoV [1], [2]. SARS-CoV, like other coronaviruses, is a positive sense, single-stranded enveloped RNA virus with a huge 29.7 Kb genome [3]. Its genome comprises of 14 ORFs which encode non-structural genes, structural genes and several unique group specific accessory proteins namely 3a, 3b, 6, 7a, 7b, 8a, 8b and 9b. [4]. Recognition of peptides derived from accessory proteins by convalescent sera of SARS-CoV infected patients [5] as well as their immuno-histochemical detection in infected VeroE6 cells and in clinical specimens [6] corroborates their expression during viral infection. However, these accessory proteins have been found dispensable for viral replication [7]. SARS-CoV accessory protein 3b is a 154 amino acid (aa) protein and has been characterized as one of the interferon antagonist encoded by SARS-CoV genome [8]. GFP tagged 3b has been reported to localize in the nucleus, nucleolus and mitochondria in cells [9], [10], [11]. A recent report delineated a unique nucleo-mitochondrial shuttling behaviour of 3b-GFP wherein 3b was found to inhibit RIG-I and MAVS induced type I interferon induction in the mitochondria [9]. Recently, we published a role of 3b in induction of ML-324 AP-1 transcriptional activity that was mediated by the activation of ERK and JNK pathways [12]. Being an interferon antagonist that is dispensable for viral replication and observing its effect on the activity of crucial host transcription factors, 3b probably plays a role in disease progression by mediating viral-host interactions, which are poorly understood. To uncover host interacting partners, of SARS-CoV 3b, we conducted a yeast two-hybrid screen of human lung cDNA library using 3b as bait. The screen identified RUNX1b (Runt related transcription factor 1, isoform b) as one of the host interacting partners of 3b. RUNX1 belongs to the RUNX family of genes which includes RUNX2 and RUNX3 additionally [13]. RUNX genes encode the subunit, which heterodimerizes with the subunit, CBF to form transcription factor CBF (Core Binding Factor) [14]. RUNX1 has a 128 aa runt domain through which it binds CBF as well as the consensus DNA element, TGT/cGGT [14], [15]. RUNX1 has three isoforms: RUNX1a, RUNX1b and RUNX1c. RUNX1a is a 250 aa protein with a runt domain. RUNX1b is a 453 aa protein and possess additional PST (proline, serine and threonine rich) region downstream to runt domain. RUNX1c differs from RUNX1b by 32 aa at N-terminus and is presumed to have similar functions in cells as RUNX1b [16]. RUNX1 LSM16 is crucially required for definitive hematopoiesis and T-lymphocyte differentiation [17], [18]. At the molecular level, RUNX1 isoforms have been shown ML-324 to regulate transcription of a number of genes including cytokines (IL2, IL3, GM-CSF etc.) and chemokines (MIP-1, CSFR etc.). Based on the yeast two-hybrid screening results, we conducted our current study with the RUNX1b isoform. In this study, we confirmed the putative interaction of 3b and RUNX1b and observed ML-324 recruitment of 3b on the RUNX1 binding element on the IL2 promoter in transiently transfected human T, jurkat cells. Further, 3b was found to increase the transactivation potential of RUNX1b on the IL2 promoter, which may partly be attributed to the enhanced ERK dependent phosphorylation of RUNX1b in 3b expressing cells. We next determined the positive effect of 3b-RUNX1b interaction on the expression of RUNX1b regulated chemokine MIP-1, reported to be upregulated in SARS-CoV infected monocyte derived dendritic cells. Thus, we report a novel interaction of SARS-CoV 3b with RUNX1b and discussed its plausible significance in SARS virus pathogenesis. Results 3b interacts with RUNX1b To identify cellular interacting ML-324 partners of SARS-CoV accessory protein 3b,.