Recombinant Saccharomyces cerevisiae Palmitoyltransferase ERF2 (ERF2)

What is Palmitoyltransferase ERF2 and what is its role in S. cerevisiae?

Palmitoyltransferase ERF2 (encoded by the YLR246W gene) is a 41-kDa integral membrane protein in Saccharomyces cerevisiae that facilitates the palmitoylation of Ras proteins. It functions as part of a heterodimeric complex with ERF4/SHR5, catalyzing the transfer of palmitate from palmitoyl-CoA to specific cysteine residues in substrate proteins, particularly Ras2. This post-translational modification is crucial for proper subcellular localization of Ras to the plasma membrane .

ERF2 contains four predicted transmembrane segments and a characteristic DHHC (Asp-His-His-Cys) motif within a cysteine-rich domain (CRD). The protein is primarily localized to the endoplasmic reticulum, where it performs its enzymatic function as part of the Ras protein processing pathway . Strains lacking ERF2 show reduced Ras palmitoylation and partial mislocalization of Ras proteins to the vacuole, demonstrating its critical role in maintaining proper Ras trafficking and function .

What is the catalytic mechanism of ERF2-mediated palmitoylation?

The palmitoylation reaction catalyzed by ERF2·ERF4 occurs through a two-step process:

• Autopalmitoylation: First, the enzyme forms a palmitoyl-ERF2 intermediate by transferring the palmitate group from palmitoyl-CoA to the cysteine residue (Cys-203) in the DHHC motif of ERF2 .

• Palmitoyl transfer: Subsequently, the palmitoyl group is transferred from the enzyme to a cysteine residue on the target substrate (e.g., Cys-318 in Ras2) .

In the absence of a protein substrate, the palmitoyl-ERF2 species undergoes hydrolysis, releasing the palmitate. This creates a continuous cycle of autopalmitoylation and hydrolysis. When Ras substrate is present, the rate of hydrolysis decreases by approximately 90%, favoring the transfer of palmitate to the substrate . The reaction is pH-dependent with optimal activity around pH 7.2, suggesting the involvement of a histidine residue (likely His-201) as a critical ionizable group in the catalytic mechanism .

Table 1: Key observations about ERF2 palmitoylation mechanism

What structural features characterize the ERF2 protein and how do they relate to function?

ERF2 contains several key structural features essential for its palmitoyltransferase activity:

Transmembrane domains

The protein contains four predicted transmembrane (TM) segments that anchor it to the ER membrane . The functional regions between TM2 and TM3 form a cytoplasmic loop containing the catalytic domain.

DHHC-CRD domain

The most crucial feature is the DHHC (Asp-His-His-Cys) motif within a cysteine-rich domain (CRD) located in the cytoplasmic loop between transmembrane segments 2 and 3 . This domain forms an atypical zinc finger structure with the consensus sequence CX₂CX₉HCX₂C (where C represents cysteine, H represents histidine, and X represents any amino acid) .

Catalytic residues

Multiple conserved residues within and around the DHHC-CRD have been identified through mutational analysis:

• Cys-203 in the DHHC motif is the site of autopalmitoylation

• His-201 is essential for palmitate transfer but not autopalmitoylation

• Other conserved residues including Cys-175, Cys-178, Arg-185, His-188, Cys-189, Cys-192, Cys-195, Asp-200, His-202, Cys-209, and F218 are critical for function

Mutations in these residues can selectively affect different aspects of the catalytic mechanism, such as autopalmitoylation rate, steady-state levels of palmitoyl-ERF2, or transfer to the substrate .

What experimental approaches can be used to measure ERF2 palmitoyltransferase activity?

Several complementary assays have been developed to measure different aspects of ERF2 palmitoyltransferase activity:

Autopalmitoylation assay

This assay measures the formation of palmitoyl-ERF2 intermediate by incubating purified ERF2·ERF4 complex with [³H]palmitoyl-CoA. The reaction is terminated at various time points, and the products are separated by SDS-PAGE. The amount of [³H]palmitate incorporated into ERF2 is quantified by fluorography or scintillation counting .

Palmitate transfer assay

To measure transfer to substrate, purified ERF2·ERF4 complex is incubated with [³H]palmitoyl-CoA and a substrate protein (e.g., GST-mCherry-Ras2CT35). The reaction products are analyzed by SDS-PAGE and fluorography to detect [³H]palmitate incorporation into the substrate .

CoASH production assay

This coupled enzyme assay indirectly monitors the turnover of palmitoyl-enzyme species by measuring the rate of CoASH release. This approach allows for kinetic analysis of the enzymatic reaction .

TLC-based hydrolysis assay

Thin-layer chromatography (TLC) can be used to separate and quantify [³H]palmitoyl-CoA, CoASH, and free [³H]palmitate, allowing researchers to monitor both the consumption of palmitoyl-CoA and the release of free palmitate through hydrolysis .

In vivo complementation assay

This genetic approach uses a yeast strain with a palmitoylation-dependent RAS2 allele to test the functionality of ERF2 mutants. Complementation is assessed by growth on media containing 5-FOA, which selects against cells retaining a URA3-marked plasmid carrying wild-type RAS2 .

What phenotypes are observed in erf2Δ strains?

Deletion of the ERF2 gene in S. cerevisiae results in several observable phenotypes:

• Viability: erf2Δ strains are viable, indicating that ERF2 is not essential for yeast survival under standard laboratory conditions .

• Ras localization defects: Ras2 proteins expressed in erf2Δ strains show partial mislocalization to the vacuole instead of proper localization to the plasma membrane .

• Reduced palmitoylation: Ras2 proteins show significantly reduced levels of palmitoylation, confirming ERF2's role in this post-translational modification .

• Synthetic growth defects: erf2Δ strains exhibit synthetic growth defects when combined with RAS2 deletion, suggesting functional interaction between these pathways .

• Suppression of hyperactive Ras: The erf2Δ mutation partially suppresses the heat shock sensitivity phenotype resulting from expression of the hyperactive RAS2(V19) allele .

These phenotypes collectively demonstrate the importance of ERF2 in the Ras signaling pathway, particularly in maintaining proper subcellular localization and function of Ras proteins through palmitoylation.

How can recombinant ERF2 be expressed and purified for in vitro studies?

Successful expression and purification of functional recombinant ERF2 requires specific strategies to maintain protein stability and activity:

Expression system

The most effective system for producing functional ERF2 is expression in Saccharomyces cerevisiae itself, as demonstrated in multiple studies . This homologous expression system ensures proper folding and post-translational modifications of the membrane protein. Heterologous expression in E. coli has proven challenging due to the multiple transmembrane domains.

Co-expression with ERF4

Critical to obtaining functional enzyme is the co-expression of ERF2 with its partner protein ERF4/SHR5, as they form a functional heterodimeric complex . Expression of ERF2 alone often results in unstable or non-functional protein.

Purification strategy

A typical purification protocol involves:

• Epitope tagging (often FLAG tag at the N-terminus rather than C-terminus, which reduces activity)

• Detergent solubilization of membranes (mild detergents to preserve protein-protein interactions)

• Affinity chromatography using the epitope tag

• Optional size exclusion chromatography to ensure complex purity

Storage conditions

Purified ERF2·ERF4 complex is typically stored in buffer containing 50% glycerol at -20°C or -80°C for extended storage . Repeated freeze-thaw cycles should be avoided, and working aliquots can be stored at 4°C for up to one week .

What are the key residues in the DHHC-CRD domain that affect ERF2 function?

Extensive mutational analysis has identified critical residues in the DHHC-CRD domain that affect different aspects of ERF2 function:

Catalytic core residues (completely abolish function)

• Cys-203 (in DHHC motif): The primary site of autopalmitoylation; mutation to serine completely abolishes both autopalmitoylation and transfer activity

• His-201 (in DHHC motif): Essential for palmitate transfer but not autopalmitoylation; H201A mutant can form palmitoyl-ERF2 intermediate but cannot transfer palmitate to Ras2

• Asp-200 (in DHHC motif): Required for function; D200A mutation causes loss of activity in vivo

• His-202 (in DHHC motif): Required for function; H202A mutation causes loss of activity in vivo

Zinc-coordinating residues (structural importance)

Mutations in the atypical zinc finger motif (CX₂CX₉HCX₂C) destabilize the protein:

• Cys-175, Cys-178, His-188, Cys-189, Cys-192, and Cys-195

Residues affecting reaction kinetics

• Arg-185: R185A mutant shows reduced steady-state levels of palmitoyl-ERF2 but can still transfer palmitate to Ras2

• Cys-209: C209S mutant shows increased hydrolysis rates and reduced steady-state levels of palmitoyl-ERF2

• Phe-218: F218S mutant shows delayed autopalmitoylation but can still transfer palmitate to Ras2

These studies demonstrate that different residues within the DHHC-CRD domain play distinct roles in the catalytic mechanism, with some affecting autopalmitoylation, others affecting palmitate transfer, and some affecting the structural stability of the domain.

How does the ERF2-ERF4 complex specifically recognize and palmitoylate Ras proteins?

The specificity of the ERF2-ERF4 complex for Ras substrates involves several molecular determinants:

Substrate recognition elements

While the DHHC-CRD domain is sufficient for autopalmitoylation, the N- and C-terminal regions of ERF2 are critical for substrate recognition and palmitate transfer to Ras2. Deletion of either region dramatically decreases palmitate transfer to Ras2 both in vitro and in vivo, despite retaining autopalmitoylation activity .

Target site context

The ERF2-ERF4 complex specifically palmitoylates yeast Ras2 on Cys-318, which is adjacent to the farnesylated cysteine (Cys-319) at the C-terminus . This indicates that prior farnesylation may create a recognition motif for the ERF2-ERF4 complex.

Substrate binding effects

Binding of the Ras2 substrate to the ERF2-ERF4 complex influences the enzyme's activity in several ways:

• Increases the production of CoASH, suggesting enhanced enzyme activity

• Reduces the rate of hydrolysis of the palmitoyl-ERF2 intermediate by approximately 90%

• Potentially stabilizes the enzyme in a more active conformation

These observations suggest that substrate binding induces conformational changes in the enzyme that optimize it for palmitate transfer while minimizing wasteful hydrolysis.

How do mutations in the DHHC-CRD domain affect the palmitoylation reaction kinetics?

Mutations in the DHHC-CRD domain have diverse effects on the kinetics of the palmitoylation reaction, revealing insights into the catalytic mechanism:

Effects on autopalmitoylation kinetics

• C203S mutation: Completely abolishes the initial burst in autopalmitoylation, confirming this residue as the site of thioesterification during autopalmitoylation

• H201A mutation: Eliminates the initial burst in autopalmitoylation but still allows formation of palmitoyl-ERF2 at a much slower rate

• F218S mutation: Shows a significant delay in reaching steady-state levels of palmitoyl-ERF2 (>60 minutes compared to 30 seconds for wild-type)

• R185A and C209S mutations: Exhibit reduced steady-state levels of palmitoyl-ERF2 (approximately 10% for R185A and 50% for C209S compared to wild-type)

Effects on hydrolysis rates

• R185A and C209S mutations: Show increased rates of hydrolysis of the palmitoyl-ERF2 intermediate, which correlates with their reduced steady-state levels of palmitoyl-ERF2

• H201A mutation: Exhibits greatly reduced hydrolysis of the palmitoyl-enzyme intermediate, showing little to no turnover

Effects on palmitate transfer to Ras2

• H201A and C203S mutations: Cannot transfer palmitate to Ras2, demonstrating complete loss of this function

• R185A, C209S, and F218S mutations: Despite showing altered autopalmitoylation kinetics, these mutants can still transfer palmitate to Ras2 at rates similar to wild-type after 60 minutes of incubation

These differential effects on the various steps of the reaction suggest that the DHHC-CRD domain contains distinct elements responsible for autopalmitoylation, hydrolysis, and transfer activities.

What is known about the evolutionary conservation of ERF2 and its homologs across eukaryotes?

ERF2 belongs to a family of DHHC-containing palmitoyltransferases that are widely conserved across eukaryotes:

Structural conservation

The defining feature of this family is the DHHC-CRD domain, which is highly conserved from yeast to humans. Most family members contain:

• Multiple transmembrane domains

• A cytoplasmic DHHC-CRD domain between transmembrane segments

• The characteristic DHHC motif within a cysteine-rich domain

Functional conservation

Putative members of the ERF2 family have been identified in diverse eukaryotes including yeast, plants, worms, insects, and mammals, suggesting a conserved role in protein palmitoylation and localization across eukaryotic evolution .

Variations within the family

• A small subset of DHHC proteins lack the conserved cysteine and histidine residues that constitute the CRD (e.g., Akr1, Akr2, and Pfa5 in yeast and DHHC22 in humans)

• These non-CRD DHHC proteins still function as palmitoyltransferases, suggesting that zinc binding by the CRD is not absolutely required for palmitoyltransferase activity

Substrate specificity

Different DHHC proteins across species show distinct substrate specificities, with ERF2 being relatively specific for Ras proteins in yeast. The molecular basis for this substrate specificity appears to involve regions outside the DHHC-CRD domain, particularly the N- and C-terminal regions .

Reaction pathway

In the absence of a protein substrate, the palmitoyl-ERF2 intermediate undergoes hydrolysis, releasing free palmitate. This creates a futile cycle of autopalmitoylation followed by hydrolysis, maintaining the enzyme in a continuously palmitoylated state ready for substrate arrival .

Effect of substrate binding

When Ras2 substrate is present, the rate of hydrolysis decreases dramatically (by approximately 90%, from 0.05 pmol/s to 0.01 pmol/s), favoring the transfer of palmitate to the substrate over wasteful hydrolysis .

Mutations affecting hydrolysis

Certain mutations in the DHHC-CRD domain specifically affect hydrolysis rates:

• R185A and C209S mutations increase the rate of hydrolysis, which correlates with reduced steady-state levels of palmitoyl-ERF2

• H201A mutation greatly reduces hydrolysis, resulting in accumulation of the palmitoyl-enzyme intermediate

These findings suggest that the balance between hydrolysis and transfer is a key regulatory aspect of ERF2 function, with the enzyme evolved to minimize wasteful hydrolysis when substrate is present.

What biochemical model explains the acid-base catalysis in the ERF2 palmitoylation reaction?

Based on mutational and kinetic analyses, a detailed acid-base catalysis model has been proposed for the ERF2 palmitoylation mechanism:

pH dependence

The autopalmitoylation reaction is regulated by one ionization group with a pKa of 7.2, suggesting the involvement of a histidine residue in the catalytic mechanism .

Role of His-201

His-201 in the DHHC motif is proposed to act as a base that extracts a proton from the substrate cysteine (Cys-318 of Ras2), producing a thiolate anion that can act as a nucleophile to attack the palmitoyl-enzyme thioester bond .

Two His/Cys dyads

The enzyme appears to contain two His/Cys dyads:

• The DHHC motif itself (His-201/Cys-203), involved in autopalmitoylation

• A second dyad involved in the transfer reaction

The association of the Ras2 substrate may be required to "neutralize" the His-201-containing dyad so that it cannot interfere with the formation of palmitoyl-ERF2 .

Catalytic mechanism

The proposed sequence is:

• His-201 acts as a base to deprotonate Cys-203, allowing it to attack palmitoyl-CoA

• The resulting palmitoyl-Cys-203 thioester is then attacked by either water (hydrolysis) or the deprotonated Cys-318 of Ras2 (transfer)

• For transfer to occur, His-201 must again act as a base to deprotonate Cys-318 of Ras2