HYPK promotes the activity of the <i>N</i>^α-acetyltransferase A complex to determine proteostasis of nonAc-X²/N-degron-containing proteins.

Abstract

In humans, the Huntingtin yeast partner K (HYPK) binds to the ribosome-associated <i>N</i>^α-acetyltransferase A (NatA) complex that acetylates ~40% of the proteome in humans and <i>Arabidopsis thaliana</i>. However, the relevance of <i>Hs</i>HYPK for determining the human N-acetylome is unclear. Here, we identify the <i>At</i>HYPK protein as the first in vivo regulator of NatA activity in plants<i>. At</i>HYPK physically interacts with the ribosome-anchoring subunit of NatA and promotes N^α-terminal acetylation of diverse NatA substrates. Loss-of-<i>At</i>HYPK mutants are remarkably resistant to drought stress and strongly resemble the phenotype of NatA-depleted plants. The ectopic expression of <i>Hs</i>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-X²/N-degron. This N-degron targets many nonacetylated NatA substrates for degradation by the ubiquitin-proteasome system.