Component of the sequence-specific heterotrimeric transcription factor (NF-Y) which specifically recognizes a 5'-CCAAT-3' box motif found in the promoters of its target genes. NF-Y can function as both an activator and a repressor, depending on its interacting cofactors (By similarity). Heterotrimeric transcription factor composed of three components, NF-YA, NF-YB and NF-YC. NF-YB and NF-YC must interact and dimerize for NF-YA association and DNA binding. Interacts with C1QBP (By similarity). Can be divided into 3 domains: the weakly conserved A domain, the highly conserved B domain thought to be involved in subunit interaction and DNA binding, and the Glu-rich C domain. Monoubiquitination at Lys-140 plays an important role in transcriptional activation by allowing the deposition of histone H3 methylations as well as histone H2B monoubiquitination at 'Lys-121'. Belongs to the NFYB/HAP3 subunit family.