Plays a major role in antagonizing the type I IFN-mediated antiviral response by degrading or inhibiting multiple cellular factors required for either IFN induction or response pathways (PubMed:15047850). Acts cooperatively with NS2 to repress activation and nuclear translocation of host IFN-regulatory factor IRF3 (PubMed:15827150). Also disrupts the association of IRF3 with CREBBP (PubMed:21632562). Interacts with host mitochondrial-associated membrane (MAM) MAVS and prevents the interaction with DDX58/RIG-I (PubMed:22383950) (Probable). Interacts with TRIM25 to suppress TRIM25-mediated DDX58 ubiquitination and thereby DDX58-MAVS interaction (PubMed:30558248). Together with NS2, participates in the proteasomal degradation of host STAT2, IRF3, IRF7, TBK1 and DDX58/RIG-I through a NS-degradasome involving CUL2 and Elongin-C (PubMed:17251292, PubMed:23877405). The degradasome requires an intact mitochondrial MAVS (PubMed:23877405). Decreases the levels of host TRAF3 and IKBKE/IKK-epsilon (PubMed:19625398). As functions other than disruptions of the type I IFN-mediated antiviral signaling pathways, induces host SOCS1 and SOCS3 expression (PubMed:24480984, PubMed:26557722). Suppresses premature apoptosis by an NF-kappa-B-dependent, interferon-independent mechanism and thus facilitates virus growth (PubMed:17151097). Additionally, NS1 may serve some inhibitory role in viral transcription and RNA replication (PubMed:9445048). Suppresses proliferation and activation of host CD103+ CD8+ cytotoxic T-lymphocytes and Th17 helper T-lymphocytes (PubMed:21533073). Monomer (PubMed:8864205). Homomultimer (PubMed:21795342, PubMed:8864205). Heteromultimer with NS2 (PubMed:21795342). Interacts with the matrix protein M (PubMed:8864205). Interacts with host ELOC and CUL2; this interaction allows NS1 to form an active E3 ligase with ELOC and CUL2 (PubMed:17251292). Interacts with host IRF3; this interaction leads to the disrupted association of IRF3 with CREBBP and thus reduced binding of IRF3 to the IFN-beta promoter (PubMed:21632562). Interacts with host MAVS; this interaction prevents MAVS binding to DDX58 and inhibits signaling pathway leading to interferon production (PubMed:22383950). Interacts with host MAP1B/microtubule-associated protein 1B (PubMed:21795342). Interacts with host TRIM25 (via SPRY domain); this interaction suppresses DDX58 ubiquitination and results in decreased interaction between DDX58 and MAVS (PubMed:30558248). Most NS1 resides in the mitochondria as a heteromer with NS2. N-terminus is important for IKBKE/IKK-epsilon reduction (PubMed:21795342). The DNLP motif has IFN suppressive functions like binding to host MAP1B (PubMed:21795342). Belongs to the pneumovirus non-structural protein 1 family.