When associated with NACA, prevents inappropriate targeting of non-secretory polypeptides to the endoplasmic reticulum (ER). Binds to nascent polypeptide chains as they emerge from the ribosome and blocks their interaction with the signal recognition particle (SRP), which normally targets nascent secretory peptides to the ER. BTF3 is also a general transcription factor that can form a stable complex with RNA polymerase II. Required for the initiation of transcription (By similarity). Part of the nascent polypeptide-associated complex (NAC), which is a heterodimer of NACA and BTF3 (via NAC-A/B domains). NAC associates with ribosomes through the BTF3/NACB subunit. Both subunits can contact nascent polypeptide chains (By similarity). The heterodimer with NACA is cytoplasmic. Belongs to the NAC-beta family.