Recombinant Human Probable palmitoyltransferase ZDHHC16 (ZDHHC16)

What is ZDHHC16 and what is its primary function?

ZDHHC16 is a member of the DHHC palmitoyltransferase family, a group of multispanning transmembrane proteins that catalyze S-palmitoylation. S-palmitoylation involves the addition of an acyl chain (generally C16 in mammals) to cysteine residues of target proteins via a thioester bond. This modification alters protein hydrophobicity and can influence protein-membrane interactions, conformation, trafficking, stability, and activity .

The primary function of ZDHHC16 identified in research is its role as an upstream palmitoyltransferase that controls ZDHHC6, another DHHC family enzyme. This relationship represents the first documented palmitoylation cascade, conceptually similar to phosphorylation cascades like the MAP kinase pathway . Through this activity, ZDHHC16 indirectly regulates the palmitoylation of ZDHHC6 substrates, including key proteins of the endoplasmic reticulum such as calnexin and transferrin receptor .

Where is ZDHHC16 localized in cells?

Under overexpression conditions, ZDHHC16 localizes to both the endoplasmic reticulum (ER) and the Golgi apparatus . This dual localization is important for understanding its functional context, as these organelles are central hubs for protein synthesis, folding, modification, and trafficking. The positioning of ZDHHC16 at these locations enables it to interact with and modify ZDHHC6, which primarily functions at the ER .

How does ZDHHC16 affect the palmitoylation status of its downstream targets?

ZDHHC16 affects downstream targets through a cascade mechanism. It directly palmitoylates ZDHHC6 on three cysteine residues (Cys-328, Cys-329, and Cys-343), which in turn modulates ZDHHC6's ability to palmitoylate its own substrates .

The palmitoylation status of ZDHHC6 target proteins is significantly impacted by ZDHHC16 activity. For example, calnexin palmitoylation was substantially reduced in ZDHHC16 knockout cells, as demonstrated through Acyl-RAC experiments . Conversely, overexpression of ZDHHC16 enhanced calnexin palmitoylation, confirming that ZDHHC16-mediated palmitoylation modulates ZDHHC6 activity toward its substrates .

Is ZDHHC16 itself subject to palmitoylation?

Unlike many proteins in the palmitoylation cascade, ZDHHC16 itself is not palmitoylated in HeLa cells. This has been conclusively demonstrated through both ³H-palmitate incorporation assays and Acyl-RAC techniques . This characteristic distinguishes ZDHHC16 from ZDHHC6 and places it at the apex of the palmitoylation cascade, functioning as an initiator rather than an intermediate in the pathway .

What are the molecular mechanisms of the ZDHHC16-ZDHHC6 palmitoylation cascade?

The ZDHHC16-ZDHHC6 palmitoylation cascade involves several coordinated molecular events. ZDHHC16 catalyzes the palmitoylation of ZDHHC6 on three specific cysteine residues: Cys-328, Cys-329, and Cys-343 . These modifications occur in a dynamic manner, with each site exhibiting different effects on ZDHHC6 stability and activity.

The palmitoylation of Cys-328 particularly stands out as it significantly accelerates ZDHHC6 protein turnover by promoting ubiquitination and subsequent degradation through the proteasome pathway . Experimentally, this has been demonstrated through the rescue of ZDHHC6 degradation by the proteasome inhibitor MG132 .

Depalmitoylation in this cascade is primarily mediated by the Acyl Protein Thioesterase APT2 (encoded by the LYPLA2 gene), which counteracts ZDHHC16's activity . The balance between ZDHHC16-mediated palmitoylation and APT2-mediated depalmitoylation creates a dynamic cycle that regulates ZDHHC6 levels and activity .

How do the different palmitoylation sites on ZDHHC6 influence its function and stability?

The three palmitoylation sites on ZDHHC6 have distinct effects on its function and stability:

• Cys-328 palmitoylation: Significantly accelerates protein turnover, with mathematical modeling predicting a half-life reduction from approximately 40 hours (unpalmitoylated) to about 5 hours when only this site is palmitoylated . When all three sites are palmitoylated, the half-life decreases further to approximately 0.3 hours. This site is also crucial for ZDHHC6 activity toward its substrates .

• Cys-329 palmitoylation: Has a stabilizing effect on ZDHHC6, with mathematical models predicting a half-life exceeding 100 hours when only this site is palmitoylated .

• Cys-343 palmitoylation: Has a moderately destabilizing effect, with a predicted half-life of about 18 hours when only this site is palmitoylated .

Through site-specific mutagenesis studies, researchers found that palmitoylation of Cys-328 confers the highest activity to ZDHHC6, while the CAA, CCA, and CAC mutants (with Cys-328 intact) showed substantial activity toward substrates compared to ACC, AAC, and ACA mutants .

What mathematical modeling approaches have been used to understand ZDHHC16-ZDHHC6 dynamics?

A sophisticated mathematical model was developed to understand the dynamics of the ZDHHC6 palmitoylation system, which includes eight possible species of ZDHHC6 based on the occupancy of the three palmitoylation sites . The model was constructed as an open system, accounting for:

• Protein synthesis resulting in an unfolded species (U)

• Folding to produce the unpalmitoylated C⁰⁰⁰ species

• Sequential palmitoylation events mediated by ZDHHC16

• Depalmitoylation events catalyzed by APT2

• First-order degradation with different rate constants for each species

The model incorporated competition between the three sites in the enzymatic kinetics, similar to previous models of calnexin palmitoylation . Parameter estimation employed a stochastic optimization method, generating a population of 10,000 models from which 152 were selected based on how accurately they fitted the experimental data .

Calibration datasets included:

• Metabolic ³⁵S-Cys/Met pulse-chase experiments for protein turnover

• ³H-palmitate incorporation data

• ³H-palmitate release data for wild-type and mutant ZDHHC6

This mathematical approach provided critical insights, such as the prediction that palmitoylation of Cys-328 had a destabilizing effect, which was subsequently confirmed experimentally by increasing the pulse labeling time from 20 minutes to 2 hours .

How does ZDHHC16 overexpression affect cellular ZDHHC6 dynamics?

Overexpression of ZDHHC16 dramatically impacts ZDHHC6 dynamics in several ways:

• Accelerated turnover: ZDHHC16 overexpression reduces the half-life of ZDHHC6 from approximately 16 hours to about 2 hours .

• Increased palmitoylation flux: Stochastic simulations predicted that ZDHHC16 overexpression would drastically increase the dynamics of the network, with all ZDHHC6 molecules rapidly cycling through all possible palmitoylation states with extremely short residence times in each state .

• Shift in species distribution: C⁰¹¹ (palmitoylated at Cys-329 and Cys-343 but not at Cys-328) emerges as the hub of the system under high ZDHHC16 activity conditions. Analysis of 10,000 simulations revealed 22,000 palmitoylation-depalmitoylation events emanating from C⁰¹¹ compared to only 4,000 events between C⁰⁰⁰ and C¹⁰⁰ .

• Protein stability: C⁰¹¹ has the slowest turnover rate of all ZDHHC6 species, explaining why protein levels remain stable despite the increased flux through different palmitoylation states .

• Enhanced substrate palmitoylation: ZDHHC16 overexpression enhances ZDHHC6-mediated palmitoylation of substrates such as calnexin, demonstrating that the palmitoylation cascade has functional consequences for downstream targets .

What techniques are used to detect and measure ZDHHC16-mediated palmitoylation?

Several complementary techniques have been employed to detect and measure ZDHHC16-mediated palmitoylation:

• ³H-palmitate metabolic labeling: This approach involves incubating cells with radioactive palmitate (³H-palmitate) followed by immunoprecipitation of the protein of interest and detection by autoradiography. This method was used to demonstrate that ZDHHC16 is not itself palmitoylated and to measure palmitate incorporation into ZDHHC6 and its substrates .

• Acyl-RAC (Resin-Assisted Capture): This non-radioactive method involves treating samples with hydroxylamine to cleave thioester bonds, followed by capture of the newly exposed thiols on thiol-reactive resin. It was used to confirm the palmitoylation status of ZDHHC16 and to assess calnexin palmitoylation in ZDHHC16 knockout cells .

• PEGylation analysis: This technique involves the reaction of free thiols with polyethylene glycol (PEG) maleimide, causing a mobility shift that can be detected by Western blotting. It was used to compare ZDHHC6 palmitoylation in tissue culture cells versus mouse tissues, revealing more pronounced palmitoylation in vivo .

• Pulse-chase experiments: These were conducted using both ³H-palmitate (to monitor palmitate turnover) and ³⁵S-Cys/Met (to monitor protein turnover). These experiments revealed that ZDHHC6 exhibits rapid turnover of palmitate, with 50% loss within approximately 1 hour .

How can genetic manipulation be used to study the ZDHHC16-ZDHHC6 relationship?

Several genetic manipulation approaches have been employed to study the ZDHHC16-ZDHHC6 relationship:

• siRNA screening: A systematic siRNA-based screen targeting all human DHHC enzymes identified ZDHHC16 as responsible for ZDHHC6 palmitoylation .

• CRISPR-Cas9 knockout: HAP1 cells with specific DHHC enzymes knocked out using CRISPR-Cas9 technology confirmed that ZDHHC6 palmitoylation was lost exclusively in ZDHHC16 knockout cells .

• Overexpression studies: Overexpression of DHHC enzymes, particularly ZDHHC16, helped identify it as the enzyme responsible for ZDHHC6 palmitoylation and demonstrated its effects on ZDHHC6 stability and function .

• Site-directed mutagenesis: Mutation of specific cysteine residues (C328A, C329A, C343A) in different combinations allowed researchers to determine the role of each palmitoylation site in ZDHHC6 function and stability .

• Gene silencing of depalmitoylating enzymes: siRNA targeting of LYPLA2 (encoding APT2) and LYPLA1 (encoding APT1) revealed that only APT2 affects ZDHHC6 palmitoylation dynamics .

What experimental approaches are used to study protein-protein interactions involving ZDHHC16?

The interaction between ZDHHC16 and ZDHHC6 has been studied using co-immunoprecipitation experiments. Following transient overexpression of myc-tagged ZDHHC6 and FLAG-tagged ZDHHC16, researchers demonstrated that the two enzymes physically interact . This approach involves:

• Transfection of cells with tagged constructs

• Cell lysis under conditions that preserve protein-protein interactions

• Immunoprecipitation using antibodies against one of the tags

• Western blot analysis to detect the co-precipitated partner protein

Additionally, genetic interaction between ZDHHC6 and ZDHHC16 was observed through quantitative PCR analysis, which showed that ZDHHC6 silencing or knockout led to an increase in ZDHHC16 mRNA levels, indicating a compensatory mechanism .

How is mathematical modeling integrated with experimental data in ZDHHC16 research?

The integration of mathematical modeling with experimental data in ZDHHC16 research involves an iterative process:

• Model development: A mathematical model was constructed based on initial experimental observations, including the identification of three palmitoylation sites on ZDHHC6 and the enzymes involved in palmitoylation (ZDHHC16) and depalmitoylation (APT2) .

• Parameter estimation: Multiple datasets were used simultaneously to calibrate the model, including:

  • Metabolic ³⁵S-Cys/Met pulse-chase data for wild-type and mutant ZDHHC6

  • ³H-palmitate incorporation data

  • ³H-palmitate release data

• Model validation: Additional experiments not used in calibration were employed to validate the model's predictions. The model accurately predicted the results of these experiments, confirming its reliability .

• Prediction and experimental confirmation: The model generated novel predictions, such as the destabilizing effect of Cys-328 palmitoylation, which were subsequently tested and confirmed experimentally. For instance, the model predicted that increasing pulse labeling time from 20 minutes to 2 hours would make differences in turnover rates more apparent .

• Stochastic simulations: These were used to analyze the dynamics of the system under different conditions, such as ZDHHC16 overexpression, revealing shifts in the distribution of ZDHHC6 species and identifying C⁰¹¹ as a hub in the system .