Acylation in trypanosomatids: an essential process and potential drug target

Abstract

Fatty acylation--the addition of fatty acid moieties such as myristate and palmitate to proteins--is essential for the survival, growth, and infectivity of the trypanosomatids: Trypanosoma brucei, Trypanosoma cruzi, and Leishmania. Myristoylation and palmitoylation are critical for parasite growth, targeting and localization, and the intrinsic function of some proteins. The trypanosomatids possess a single N-myristoyltransferase (NMT) and multiple palmitoyl acyltransferases, and these enzymes and their protein targets are only now being characterized. Global inhibition of either process leads to cell death in trypanosomatids, and genetic ablation of NMT compromises virulence. Moreover, NMT inhibitors effectively cure T. brucei infection in rodents. Thus, protein acylation represents an attractive target for the development of new trypanocidal drugs.

Keywords: myristoylation; palmitoylation; trafficking; virulence.

Figure 1

Myristoylation and palmitoylation. Myristoylation: N-myristoyl transferase (NMT) binds to myristoyl-CoA and the recognition sequence (Figure 2) of its substrate protein. Nucleophilic substitution of the N-terminal glycine amine with myristate leads to the production of myristoylated protein and CoA-SH. Palmitoylation: palmitoylation is catalyzed by the enzyme palmitoyl acyltransferase (PAT), which is reversed by the action of a thioesterase. PAT binds to palmitoyl-CoA and the recognition sequence (Figure 2) of its substrate protein via its DHHC catalytic site. PAT first becomes autopalmitoylated and then transfers the palmitate to the substrate protein to generate palmitoylated protein and CoA-SH.

Figure 2

Recognition sequences for myristoylation and palmitoylation. Protein myristoylation occurs on the N-terminal glycine residue at position 2 (red), with additional aspects of the sequence motif indicated in positions 3-9, where X represents any amino acid. Protein palmitoylation occurs at three types of sites, which may be located throughout the polypeptide sequence: Type I (XCCX), Type II (CXXC), or Type III (Random) pattern, where X represents any amino acid. The X amino acid residues in palmitoylation sites are preferentially neutral > basic > acidic.

Figure 3

Hypothetical trafficking pattern of dually acylated proteins. Hypothetical pathway for an exemplar dually acylated myristoylated/palmitoylated trypanosomatid protein, calflagin/FCaBP, based in part on data in [71] and [72]. (1) Newly-synthesized protein (blue) is myristoylated by NMT and (2) associates with vesicles in the Golgi, perhaps having a distinct lipid composition specifying flagellar pocket (FP) targeting. (3) The vesicles traffick to and fuse with the flagellar pocket. (4) If the protein is not palmitoylated by PAT (orange, shown here in the FP), it trafficks to the pellicular membrane. (5) If the protein is palmitoylated, it has two possible fates, depending on whether it also possesses a polybasic region (KKKK). (6) If KKKK is present, the protein enters the flagellar compartment and associates with the flagellar membrane. (7) If KKKK is absent, it continues onto the pellicular membrane. This is only one model for trafficking of one protein. Additional known and possible factors specifying trafficking of acylated proteins include protein-protein interactions, the specific lipid chemistry of distinct membrane domains, such as the flagellar membrane [79], and the location of the specific PAT.