The role of CddA and cGNAT2 in crotonylation of proteins in cyanobacteria

Summary Lysine crotonylation (Kcr) is a newly identified posttranslational modification that plays an important role in diverse biological processes; however, its distribution, function, and regulation in photosynthetic organisms remain largely unknown. Cyanobacteria are the most ancient prokaryotes capable of oxygenic photosynthesis and play a vital role in global carbon and nitrogen cycles. We examined all predicted Kcr regulatory enzymes in the model cyanobacterium Synechococcus sp. PCC 7002 (Syn7002) using total protein Kcr modification levels and enzymatic activity assays. We then used a label‐free quantitative (LFQ) proteomic approach following enrichment for crotonylated peptides to identify the endogenous substrates of these Kcr regulatory enzymes. We found that cGNAT2 functions as a lysine crotonyltransferase, whereas CddA acts as a decrotonylase. Using LFQ crotonylome analysis, we identified a total of 536 endogenous Kcr sites catalyzed by cGNAT2 and 360 candidate sites targeted by CddA, with the associated proteins predominantly involved in metabolic processes and photosynthesis. Furthermore, we validated that cGNAT2 and CddA regulate the Kcr level of the Photosystem I subunit II (PsaD). cGNAT2 and CddA may influence the structure of PsaD by modulating its Kcr status or by cumulative modification effects, thereby affecting cell growth and the efficiency of photosynthetic electron transport.