Gram-negative bacteria including Escherichia coli, Salmonella typhimurium, and Pseudomonas aeruginosa can modify the structure of lipid A in their outer membrane with 4-amino-4-deoxy-Larabinose (Ara4N). Such modification results in resistance to cationic antimicrobial peptides of the innate immune system and antibiotics such as polymyxin. ArnA is a key enzyme in the lipid A modification pathway, and its deletion abolishes both the Ara4N-lipid A modification and polymyxin resistance. ArnA is a bifunctional enzyme. It can catalyze (i) the NAD + -dependent decarboxylation of UDP-glucuronic acid to UDP-4-keto-arabinose and (ii) the N-10-formyltetrahydrofolatedependent formylation of UDP-4-amino-4-deoxy-L-arabinose. We show that the NAD + -dependent decarboxylating activity is contained in the 360 amino acid C-terminal domain of ArnA. This domain is separable from the N-terminal fragment, and its activity is identical to that of the full-length enzyme. The crystal structure of the ArnA decarboxylase domain from E. coli is presented here. The structure confirms that the enzyme belongs to the short-chain dehydrogenase/reductase (SDR) family. On the basis of sequence and structure comparisons of the ArnA decarboxylase domain with other members of the short-chain dehydrogenase/reductase (SDR) family, we propose a binding model for NAD + and UDP-glucuronic acid and the involvement of residues T 432 , Y 463 , K 467 ,R 619 , and S 433 in the mechanism of NAD + -dependent oxidation of the 4″-OH of the UDP-glucuronic acid and decarboxylation of the UDP-4-keto-glucuronic acid intermediate.In the process of establishing infections, bacteria must overcome the host defense mechanism including the bactericidal action of cationic antimicrobial peptides (CAMPs). 1 These are small, amphipathic, positively charged peptides that destroy bacteria through membrane permeabilization and constitute a phylogenetically conserved branch of the innate immune system (1-4). In the case of Gram-negative bacteria, CAMPs bind to the bacterial cell surface through electrostatic interactions with the negatively charged groups of the lipopolysaccharide (LPS), the immunogenic glycolipid in the outer membrane in Gram-negative bacteria (5,6). They then traverse to the inner membrane and form a pore, which leads to membrane permeabilization and cell death (7)(8)(9). In addition to their function as a key member of the innate immune system, CAMPs represent an important class of clinical antimicrobials. They have both intrinsic bactericidal activity and appear to enhance the activity of other antibiotics, presumably by facilitating their entry into the microbe (3,10,11 21). The N-terminal domain (residues 1-313) is similar in sequence to other enzymes involved in formyl transfer. However, the relevance of this reaction in the biosynthesis of Ara4N-lipid A is unclear. ArnA is also responsible for the C-4″ oxidation of UDP-GlcA to UDP-4-keto-glucuronic acid and its decarboxylation to yield UDP-4-keto-arabinose (boxed in Figure 1) (20). The C terminus o...