HYPK promotes the activity of the N(α)-acetyltransferase A complex to determine proteostasis of nonAc-X(2)/N-degron–containing proteins

In humans, the Huntingtin yeast partner K (HYPK) binds to the ribosome-associated N(α)-acetyltransferase A (NatA) complex that acetylates ~40% of the proteome in humans and Arabidopsis thaliana. However, the relevance of HsHYPK for determining the human N-acetylome is unclear. Here, we identify the AtHYPK protein as the first in vivo regulator of NatA activity in plants. AtHYPK physically interacts with the ribosome-anchoring subunit of NatA and promotes N(α)-terminal acetylation of diverse NatA substrates. Loss-of-AtHYPK mutants are remarkably resistant to drought stress and strongly resemble the phenotype of NatA-depleted plants. The ectopic expression of HsHYPK 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(2)/N-degron. This N-degron targets many nonacetylated NatA substrates for degradation by the ubiquitin-proteasome system.