Acts a component of the CCAT-binding factor, which is a transcriptional activator and binds to the upstream activation site (UAS2) of the CYC1 gene and other genes involved in mitochondrial electron transport and activates their expression. Recognizes the sequence 5'-CCAAT-3'. Component of the CCAT-binding factor (CBF or HAP complex II), which consists of one copy each of HAP2, HAP3, HAP4 and HAP5. The assembly of the HAP2-HAP3-HAP5 heteromer (HAP complex I) occurs in a one-step pathway and its binding to DNA is a prerequisite for the association of HAP4. Present with 2510 molecules/cell in log phase SD medium. Belongs to the NFYB/HAP3 subunit family.