The alpha and beta subunit of the nascent polypeptide-associated complex have distinct functions.
Nascent polypeptide-associated complex (NAC) is probably the first cytosolic protein to contact nascent polypeptide chains emerging from ribosomes. In this way NAC prevents inappropriate interactions with other factors. Eventually other factors involved in targeting and folding, like the Signal Recognition Particle or cytosolic chaperones, must gain access to the nascent chain. All NAC preparations to date consist of two copurifying polypeptides. Here we rigorously show that these two polypeptides, termed alpha- and betaNAC, form a very stable complex in vivo and in vitro and that a functional complex can be reconstituted from the individual subunits. A dissection of the contributions of the individual subunits to NACs function revealed that both subunits are in direct contact with nascent polypeptide chains on the ribosome and that both contribute to the prevention of inappropriate interactions. However, betaNAC alone directly binds to the ribosome and is sufficient to prevent ribosome binding to the endoplasmic reticulum membrane.
Beatrix
B
B
Cellular Biochemistry and Biophysics Program, Memorial Sloan-Kettering Cancer Center, New York, New York 10021, USA.
Sakai
H
H
Wiedmann
M
M
D000595:Amino Acid Sequence; D000818:Animals; D002460:Cell Line; D002850:Chromatography, Gel; D004285:Dogs; D007150:Immunohistochemistry; D051379:Mice; D018832:Molecular Chaperones; D008969:Molecular Sequence Data; D009696:Nucleic Acids / Q000378:metabolism; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D018271:Signal Recognition Particle / Q000378:metabolism; D015534:Trans-Activators / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*
D018832:Molecular Chaperones; D009696:Nucleic Acids; D011994:Recombinant Proteins; D018271:Signal Recognition Particle; C503637:Srp72 protein, mouse; D015534:Trans-Activators; C096099:nascent-polypeptide-associated complex