Protein S-Acyltransferase 14: A Specific Role for Palmitoylation in Leaf Senescence in Arabidopsis

Abstract

The Asp-His-His-Cys-Cys-rich domain-containing Protein S-Acyl Transferases (PATs) are multipass transmembrane proteins that catalyze S-acylation (commonly known as S-palmitoylation), the reversible posttranslational lipid modification of proteins. Palmitoylation enhances the hydrophobicity of proteins, contributes to their membrane association, and plays roles in protein trafficking and signaling. In Arabidopsis (Arabidopsis thaliana), there are at least 24 PATs; previous studies on two PATs established important roles in growth, development, and stress responses. In this study, we identified a, to our knowledge, novel PAT, AtPAT14, in Arabidopsis. Complementation studies in yeast (Saccharomyces cerevisiae) and Arabidopsis demonstrate that AtPAT14 possesses PAT enzyme activity. Disruption of AtPAT14 by T-DNA insertion resulted in an accelerated senescence phenotype. This coincided with increased transcript levels of some senescence-specific and pathogen-resistant marker genes. We show that early senescence of pat14 does not involve the signaling molecules jasmonic acid and abscisic acid, or autophagy, but associates with salicylic acid homeostasis and signaling. This strongly suggests that AtPAT14 plays a pivotal role in regulating senescence via salicylic acid pathways. Senescence is a complex process required for normal plant growth and development and requires the coordination of many genes and signaling pathways. However, precocious senescence results in loss of biomass and seed production. The negative regulation of leaf senescence by AtPAT14 in Arabidopsis highlights, to our knowledge for the first time, a specific role for palmitoylation in leaf senescence.

Progress toward Understanding Protein S-acylation: Prospective in Plants

“…For example, AtPAT3, 15, 17, and 18 are localized at ER as well as on the vesicles associated with them; AtPAT13, 20, and 22 at PM and also vesicles (Batistič, 2012). AtPAT10, 14, 16, 23, and 24 are mainly localized at Golgi, and the Golgi-localization of AtPAT10 and AtPAT14 were further confirmed in stably transformed Arabidopsis plants (Qi et al, 2013; Li et al, 2016). While the main residence of AtPAT01 and AtPAT02 are the endosomal compartments AtPAT10 and AtPAT11 were found on the tonoplast (Batistič, 2012; Qi et al, 2013; Zhou et al, 2013).…”

“…It was reported that mutation of cysteine in DHHC domain inhibits both acyl intermediate formation and acyl chain transfer activity of PATs (Mitchell et al, 2006; Gottlieb et al, 2015). Indeed, when cysteine residue in the DHHC motif of AtPAT24, AtPAT10 and AtPAT14 of Arabidopsis was mutated to alanine or serine, all 3 AtPATs lost their PAT activities (Hemsley et al, 2005; Qi et al, 2013; Li et al, 2016). The DHHC-CRD domain in Swf1 cannot be replaced by those from Pfa3, Pfa4 or Erf2, and similar results were also found for Pfa3, the DHHC-CRD of which cannot be replaced by that of Swf1 or Erf2.…”

“…For example, three T-DNA insertion mutant alleles were identified for AtPAT10 and all showed pleiotropic defects, including cell expansion, cell division, vascular patterning, fertility and salt stress in Arabidopsis (Qi et al, 2013; Zhou et al, 2013). Single mutants of atpat13 and atpat14 are semi-dwarf and show precocious leaf senescence, and the double mutant atpat13 atpat14 has even stronger phenotype than each of their parent single mutant plants (Lai et al, 2015; Li et al, 2016). …”

“…Both alanine and serine were frequently used to replace the cysteine to achieve similar results (Hemsley et al, 2005; Qi et al, 2013; Li et al, 2016). Cysteine and serine have very similar structure, when the cysteine is mutated to serine it can maintain the size and the properties of the putative S-acylated protein, in this case, serine is a better substitution for cysteine.…”

An outlook on protein S-acylation in plants: what are the next steps?

“…S-acylation is also much more common 55 that any of the other lipid based modifications of proteins with conservative estimates 56 suggesting that over 10% of the proteome, and therefore >30% of the membrane proteome, 57 may be S-acylated in eukaryotes (Hemsley et al, 2013;Martin and Cravatt, 2009;Roth et al, 58 2006). Mutants in the S-acylating enzymes themselves frequently have severe pleiotropic 59 phenotypes indicating a substantial requirement for S-acylation in plants (Hemsley et al,60 2005; Lai et al, 2015;Li et al, 2016;Qi et al, 2013). Despite these outwardly important factors 61 suggesting that S-acylation is likely to be very important in cellular protein function, very little 62 is actually known about how S-acylation is regulated, exactly how many proteins are S-acylated, how specificity of S-acylation is determined and what exactly its effects on proteins 64 are.…”