Science 359, eaao6326 (2018)

Credit: AAAS

The reversible post-translational attachment of fatty acyl groups to cysteine residues is involved in regulating a range of cellular processes including Ras signaling and intracellular trafficking. The most common acyl chain employed in this modification is the 16-carbon palmitate; its attachment (as palmitoyl-CoA) is catalyzed by polytopic integral membrane enzymes called DHHC palmitoyltransferases, owing to their conserved catalytic motifs. To gain insight into this enzyme's catalytic mechanism and its interactions with its substrates and with the membrane, Rana et al. have now determined the crystal structures of two DHHC enzymes from human and zebrafish. Because the enzyme catalyzes palmitoylation on a substrate protein's membrane-proximal cysteine residues, the active site is situated at the interface between the membrane and aqueous phase. Using the covalent inhibitor 2-bromopalmitate afforded a mimic of the acyl intermediate that is attached to the catalytic cysteine, and the structure of this complex indicates that the acyl chain is stabilized by insertion into an enzyme cavity within the membrane bilayer. Guided by this structure, mutagenesis to increase or decrease the cavity size successfully shifted the enzyme's substrate preference from palmitoyl-CoA to longer (18-carbon) or shorter (12- and 14-carbon) acyl chains, respectively. In the future, the structures of these DHHC enzymes may be useful for guiding further engineering of orthogonal enzyme–acyl-CoA pairs and for designing new small-molecule probes. CD