HYPK promotes the activity of the -acetyltransferase A complex to determine proteostasis of nonAc-X/N-degron-containing proteins.

In humans, the Huntingtin yeast partner K (HYPK) binds to the ribosome-associated -acetyltransferase A (NatA) complex that acetylates ~40% of the proteome in humans and . However, the relevance of HYPK for determining the human N-acetylome is unclear. Here, we identify the HYPK protein as the first in vivo regulator of NatA activity in plantsHYPK physically interacts with the ribosome-anchoring subunit of NatA and promotes N-terminal acetylation of diverse NatA substrates. Loss-of-HYPK mutants are remarkably resistant to drought stress and strongly resemble the phenotype of NatA-depleted plants. The ectopic expression of HYPK rescues this phenotype. Combined transcriptomics, proteomics, and N-terminomics unravel that HYPK impairs plant metabolism and development, predominantly by regulating NatA activity. We demonstrate that HYPK is a critical regulator of global proteostasis by facilitating masking of the recently identified nonAc-X/N-degron. This N-degron targets many nonacetylated NatA substrates for degradation by the ubiquitin-proteasome system.