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. Also reduces the inherent affinity of ribosomes for protein translocation sites in the ER membrane (M sites). May act as a specific coactivator for JUN, binding to DNA and stabilizing the interaction of JUN homodimers with target gene promoters (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 and contacts the ribosomal protein L23, which is positioned near the exiting site. Both subunits can contact nascent polypeptide chains. NACA may also form homodimers, and only this form binds DNA. Interacts with TBP and JUN (By similarity). The heterodimer is located mainly in the cytosol, and the homodimer in the nucleus. The positively charged inner surface of the NAC-A/B domain is crucial for NACA localization in the nucleus and DNA-binding. This region is blocked from binding nucleic acids in the heterodimeric complex by a helix region in the beta-subunit, it also displays much higher affinity for RNA than DNA (By similarity). Phosphorylation of Ser-43 by ILK during cell adhesion may promote nuclear localization. Phosphorylation of Thr-159 by GSK3B may promote proteasome mediated degradation. Belongs to the NAC-alpha family.