With ∼ 250 million clinical cases and a death toll of ∼ 0.9 million per year, malaria remains a significant public health problem in many tropical and subtropical countries. Of the four human malaria parasites, Plasmodium falciparum causes the most severe form of disease and the majority of malaria-associated mortality. Whereas recent reduction in global malaria incidence has inspired renewed hopes for malaria elimination and eradication, the malaria control campaign still encounters many challenges. In particular, the parasite is notorious for developing resistance to most currently used antimalarial drugs. Therefore, continued research towards the development of novel diagnostics and therapeutics for malaria is needed, and these efforts require a comprehensive understanding of the parasite's biology.
Asexual replication of the parasites in red blood cells (RBCs) contributes to malaria-associated morbidity and mortality. Extensive microarray and proteomic analyses have established that the intraerythrocytic developmental cycle (IDC) is governed by highly regulated transcription and translation programmes (Le Roch et al., 2003; Luah et al., 2010; Foth et al., 2011). The importance of chromatin-mediated epigenetic regulation of gene expression has been increasingly appreciated, and its roles entail many aspects of parasite biology such as cell cycle regulation, invasion and virulence (Merrick and Duraisingh, 2010; Miao et al., 2010). Correspondingly, the parasite genome encodes a large suite of chromatin-remodelling and modification enzymes, among which there are at least four lysine acetyltransferases (KATs) and three classes of lysine or histone deacetylases (KDACs or HDACs) (Horrocks et al., 2009; Miao et al., 2010). KATs catalyse the transfer of the acetyl moiety from acetyl-CoA to the ε-position of a lysine residue, whereas KDACs catalyse the removal of the acetyl group from an acetylated lysine. The reversible protein lysine acetylation is a highly regulated post-translational modification found in both prokaryotes and eukaryotes. Since the discovery of protein lysine acetylation almost five decades ago (Allfrey et al., 1964), studies of this modification have focused primarily on histones, the building units of nucleosomes. We and others have identified a multitude of covalent modifications on histones of P. falciparum, including both N-terminal acetylation and lysine acetylation (Miao et al., 2006; Salcedo-Amaya et al., 2009). Histone lysine acetylation, together with other post-translational modifications such as methylation, phosphorylation, ubiquitylation and sumoylation, profoundly affects chromatin structure and gene expression (Verdone et al., 2005; Shahbazian and Grunstein, 2007).
In addition to histone lysine acetylation, lysine acetylation occurs in cytoplasmic proteins. Recently, advancement in mass spectrometry (MS) allowed characterization of the ‘acetylomes’ in bacteria (Yu et al., 2008; Zhang et al., 2009; Wang et al., 2010), yeast (Henriksen et al., 2012), the protozoan parasite Toxoplasma gondii (Jeffers and Sullivan, 2012), plants (Finkemeier et al., 2011; Wu et al., 2011), Drosophila melanogaster (Weinert et al., 2011), rat (Lundby et al., 2012) and human cells (Kim et al., 2006; Choudhary et al., 2009; Zhao et al., 2010). From these studies, lysine acetylation has emerged as a widespread post-translational modification that may rival protein phosphorylation (Maurer-Stroh et al., 2003). Proteins with acetylated lysines participate in diverse biological functions. Particularly, lysine acetylation is highly prevalent in enzymes catalysing intermediate metabolism in both bacteria and human cells (Kim et al., 2006; Choudhary et al., 2009; Wang et al., 2010; Zhao et al., 2010). To date, studies on lysine acetylation of non-histone proteins in malaria parasites are very limited. Yet, the fact that some of the Plasmodium KATs such as PfMYST are localized in both nucleus and cytoplasm suggests that regulated protein lysine acetylation occurs in both compartments of the parasite (Miao et al., 2010). In this study, we performed a proteome-wide analysis of the P. falciparum acetylome during the IDC by immunoprecipitation (IP) with specific anti-acetyllysine antibodies and accurate MS. This screen identified 230 lysine-acetylated proteins belonging to considerably diverse functional groups, suggesting that acetylation plays important roles in regulating many cellular processes in Plasmodium. Understanding the mechanism of acetylation and its role in regulating protein functions may open a new venue for development of drugs and vaccines.