Shining a light on the dark Nt-acetylome by integrating omics data

N-terminal acetylation, catalysed by N-terminal acetyltransferases (Nats), is one of the most prevalent protein modifications and is implicated in human diseases. Yet, despite extensive COFRADIC-based proteomics, only [~]5-10% of the proteome has been interrogated, leaving the majority of the Nt-acetylome unexplored. Here, we combined all major COFRADIC datasets with sel-TRAP, a high-sensitivity, orthogonal approach for profiling co-translational Nat targets via selective ribosome purification. This integrated analysis refined the substrate specificities of NatA, NatB, and NatC/E/F and provided the most comprehensive view to date of the canonical human and yeast Nt-acetylomes. Importantly, we also uncovered hundreds of cryptic Nat substrates arising from alternative translation initiation, with unexpected Nt-proteoforms constituting a previously underappreciated source of Nat targets. Collectively, our results revealed the complex landscape of a "dark" Nt-acetylome, the characterization of which, including its functional roles in regulating protein function and in disease, remains a major challenge for future research.