RESUMO
Approximately 40% of the mammalian proteome undergoes N-terminal methionine excision and acetylation, mediated sequentially by methionine aminopeptidase (MetAP) and N-acetyltransferase A (NatA), respectively1. Both modifications are strictly cotranslational and essential in higher eukaryotic organisms1. The interaction, activity and regulation of these enzymes on translating ribosomes are poorly understood. Here we perform biochemical, structural and in vivo studies to demonstrate that the nascent polypeptide-associated complex2,3 (NAC) orchestrates the action of these enzymes. NAC assembles a multienzyme complex with MetAP1 and NatA early during translation and pre-positions the active sites of both enzymes for timely sequential processing of the nascent protein. NAC further releases the inhibitory interactions from the NatA regulatory protein huntingtin yeast two-hybrid protein K4,5 (HYPK) to activate NatA on the ribosome, enforcing cotranslational N-terminal acetylation. Our results provide a mechanistic model for the cotranslational processing of proteins in eukaryotic cells.
Assuntos
Metionina , Chaperonas Moleculares , Complexos Multienzimáticos , Processamento de Proteína Pós-Traducional , Ribossomos , Animais , Humanos , Acetilação , Domínio Catalítico , Metionil Aminopeptidases/química , Metionil Aminopeptidases/metabolismo , Modelos Moleculares , Complexos Multienzimáticos/metabolismo , Complexos Multienzimáticos/química , Acetiltransferase N-Terminal A/química , Acetiltransferase N-Terminal A/metabolismo , Ribossomos/química , Ribossomos/enzimologia , Ribossomos/metabolismo , Metionina/química , Metionina/metabolismo , Chaperonas Moleculares/química , Chaperonas Moleculares/metabolismo , Caenorhabditis elegansRESUMO
N-terminal methionine excision from newly synthesized proteins, catalyzed cotranslationally by methionine aminopeptidases (METAPs), is an essential and universally conserved process that plays a key role in cell homeostasis and protein biogenesis. However, how METAPs interact with ribosomes and how their cleavage specificity is ensured is unknown. We discovered that in eukaryotes the nascent polypeptide-associated complex (NAC) controls ribosome binding of METAP1. NAC recruits METAP1 using a long, flexible tail and provides a platform for the formation of an active methionine excision complex at the ribosomal tunnel exit. This mode of interaction ensures the efficient excision of methionine from cytosolic proteins, whereas proteins targeted to the endoplasmic reticulum are spared. Our results suggest a broader mechanism for how access of protein biogenesis factors to translating ribosomes is controlled.