Mechanism of cotranslational modification of histones H2A and H4 by MetAP1 and NatD

The replication-dependent histones H2A and H4 are among the most highly expressed proteins in eukaryotes during the S phase to ensure packaging of replicated chromosomes. Nearly all newly synthesized H2A and H4 are N-terminally acetylated by N-terminal acetyltransferase D (NatD) following excision of the initiator methionine by methionine aminopeptidases (MetAPs). These modifications influence chromatin function, but how they occur cotranslationally on these exceptionally abundant and small proteins was not understood. Here, we show that the nascent polypeptide-associated complex controls the cotranslational modification of histones H2A and H4 by recruiting NatD and the upstream enzyme MetAP1 to ribosomes. MetAP1 and NatD cooperate on the ribosome to create a confined environment for the efficient sequential modification of the nascent histone chain. Our work provides a mechanistic model for the early steps of histone maturation.