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) (By similarity). Part of the nascent polypeptide-associated complex (NAC), consisting of NACA and BTF3. NAC associates with ribosomes through the BTF3 subunit. Both subunits can contact nascent polypeptide chains (By similarity). Expressed specifically in testis and skeletal muscle. NACAL arose from a recent (40-63 million-year-old), anthropoid primate-specific retroduplication of NACA. Belongs to the NAC-alpha family.