Advanced glycation end product (AGE) crosslinking of a bacterial protein: Are AGE-modifications going undetected in our studies?

The small reactive molecules, glyoxal (GO) and methylglyoxal (MGO), are common byproducts of metabolic processes. GO and MGO are known to modify proteins, DNA, and lipids, resulting in advance glycation end products (AGEs). AGEs are linked to numerous human diseases but are found across all three domains of life due to the widespread presence of GO and MGO. Recent structural studies have revealed that an antibacterial phospholipase toxin contains a methylglyoxal-derived imidazolium crosslink (MODIC). Unlike AGEs that are associated with human diseases and protein dysfunction, crosslinking is required for the toxin's enzymatic activity, indicating that MODIC acts as a bona fide post-translational modification to promote function. The MODIC-modified toxin represents the first structure in the protein data bank with an AGE-modification. However, because GO and MGO are present in all cells, AGE-modifications are likely more prevalent than currently reported but have gone undetected. We used the toxin's MODIC structural motif to query the protein data bank for other modified proteins. This search recovered the colicin Ia pore-forming toxin. Using the deposited crystal structure and structural data for colicin Ia, we were able to model glyoxal-derived imidazolium crosslink or MODIC modifications into the electron density map, suggesting that GO/MGO modifications may indeed be more common in bacterial proteins.