Myristoylation is a lipidation modification where a
myristoyl group, derived from
myristic acid
Myristic acid (IUPAC name: tetradecanoic acid) is a common saturated fatty acid with the molecular formula CH3(CH2)12COOH. Its salts and esters are commonly referred to as myristates or tetradecanoates. It is named after the binomial name for nut ...
, is
covalent
A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...
ly attached by an
amide bond to the alpha-amino group of an
N-terminal glycine
Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid ( carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinog ...
residue.
Myristic acid is a 14-carbon saturated fatty acid (14:0) with the systematic name of ''n''-Tetradecanoic acid. This modification can be added either co-translationally or
post-translationally.
N-myristoyltransferase (NMT) catalyzes the
myristic acid
Myristic acid (IUPAC name: tetradecanoic acid) is a common saturated fatty acid with the molecular formula CH3(CH2)12COOH. Its salts and esters are commonly referred to as myristates or tetradecanoates. It is named after the binomial name for nut ...
addition reaction in the
cytoplasm
In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...
of cells.
This lipidation event is the most found type of fatty acylation
and is common among many organisms including
animal
Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
s,
plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...
s,
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
,
protozoans
and
virus
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsk ...
es. Myristoylation allows for weak protein–protein and protein–lipid interactions
and plays an essential role in membrane targeting,
protein–protein interaction
Protein–protein interactions (PPIs) are physical contacts of high specificity established between two or more protein molecules as a result of biochemical events steered by interactions that include electrostatic forces, hydrogen bonding and th ...
s and functions widely in a variety of
signal transduction pathways.
Discovery
In 1982, Koiti Titani's lab identified an "N-terminal blocking group" on the catalytic subunit of
cyclic AMP-dependent protein kinase in cows as ''n''-tetradecanoyl.
Almost simultaneously in Claude B. Klee's lab, this same N-terminal blocking group was further characterized as myristic acid.
Both labs made this discovery utilizing similar techniques:
mass spectrometry and
gas chromatography.
N-myristoyltransferase
The enzyme N-myristoyltransferase (NMT) or
glycylpeptide N-tetradecanoyltransferase is responsible for the irreversible addition of a myristoyl group to N-terminal or internal glycine residues of proteins. This modification can occur co-translationally or
post-translational
Post-translational modification (PTM) is the covalent and generally enzymatic modification of proteins following protein biosynthesis. This process occurs in the endoplasmic reticulum and the golgi apparatus. Proteins are synthesized by ribosom ...
ly. In vertebrates, this modification is carried about by two NMTs,
NMT1 and
NMT2, both of which are members of the GCN5
acetyltransferase superfamily.
Structure
The
crystal structure
In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns ...
of NMT reveals two identical subunits, each with its own myristoyl CoA binding site. Each subunit consists of a large saddle-shaped
β-sheet surrounded by
α-helices
The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues ear ...
. The symmetry of the fold is pseudo twofold. Myristoyl CoA binds at the
N-terminal portion, while the
C-terminal
The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is ...
end binds the protein.
Mechanism
The addition of the myristoyl group proceeds via a
nucleophilic addition-elimination reaction. First, myristoyl
coenzyme A (CoA) is positioned in its binding pocket of NMT so that the
carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containi ...
faces two amino acid residues,
phenylalanine 170 and
leucine
Leucine (symbol Leu or L) is an essential amino acid that is used in the biosynthesis of proteins. Leucine is an α-amino acid, meaning it contains an α- amino group (which is in the protonated −NH3+ form under biological conditions), an α- ...
171.
This polarizes the carbonyl so that there is a net positive charge on the carbon, making it susceptible to nucleophilic attack by the
glycine
Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid ( carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinog ...
residue of the protein to be modified. When myristoyl CoA binds, NMT reorients to allow binding of the peptide. The C-terminus of NMT then acts as a general base to
deprotonate
Deprotonation (or dehydronation) is the removal (transfer) of a proton (or hydron, or hydrogen cation), (H+) from a Brønsted–Lowry acid in an acid–base reaction.Henry Jakubowski, Biochemistry Online Chapter 2A3, https://employees.csbsju.ed ...
the NH
3+, activating the
amino group
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent such ...
to attack at the
carbonyl group
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containi ...
of myristoyl-CoA. The resulting
tetrahedral intermediate is stabilized by the interaction between a positively charged
oxyanion hole and the negatively charged
alkoxide anion. Free CoA is then released, causing a
conformational change in the enzyme that allows the release of the myristoylated peptide.
Co-translational vs. post-translational addition
Co-translational and post-translational covalent modifications enable proteins to develop higher levels of complexity in cellular function, further adding diversity to the
proteome
The proteome is the entire set of proteins that is, or can be, expressed by a genome, cell, tissue, or organism at a certain time. It is the set of expressed proteins in a given type of cell or organism, at a given time, under defined conditions. ...
.
The addition of myristoyl-CoA to a protein can occur during protein translation or after. During co-translational addition of the myristoyl group, the N-terminal
glycine
Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid ( carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinog ...
is modified following cleavage of the N-terminal
methionine residue in the newly forming, growing
polypeptide.
Post-translational myristoylation typically occurs following a
caspase cleavage event, resulting in the exposure of an internal glycine residue, which is then available for myristic acid addition.
Functions
Myristoylated proteins
Myristoylation molecular switch
Myristoylation not only diversifies the function of a protein, but also adds layers of regulation to it. One of the most common functions of the myristoyl group is in
membrane association and
cellular localization of the modified protein. Though the myristoyl group is added onto the end of the protein, in some cases it is sequestered within
hydrophobic
In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, t ...
regions of the protein rather than solvent exposed.
By regulating the orientation of the myristoyl group, these processes can be highly coordinated and closely controlled. Myristoylation is thus a form of "
molecular switch
A molecular switch is a molecule that can be reversibly shifted between two or more stable states. The molecules may be shifted between the states in response to environmental stimuli, such as changes in pH, light, temperature, an electric curren ...
."
Both hydrophobic myristoyl groups and "basic patches" (highly positive regions on the protein) characterize myristoyl-electrostatic switches. The basic patch allows for favorable
electrostatic interaction
Electrostatics is a branch of physics that studies electric charges at rest (static electricity).
Since classical times, it has been known that some materials, such as amber, attract lightweight particles after triboelectric effect, rubbing. ...
s to occur between the negatively charged
phospholipid heads of the membrane and the positive surface of the associating protein. This allows tighter association and directed localization of proteins.
Myristoyl-conformational switches can come in several forms.
Ligand binding
In biochemistry and pharmacology, a ligand is a substance that forms a complex with a biomolecule to serve a biological purpose. The etymology stems from ''ligare'', which means 'to bind'. In protein-ligand binding, the ligand is usually a mo ...
to a myristoylated protein with its myristoyl group sequestered can cause a
conformational change in the protein, resulting in exposure of the myristoyl group. Similarly, some myristoylated proteins are activated not by a designated ligand, but by the exchange of
GDP
Gross domestic product (GDP) is a monetary measure of the market value of all the final goods and services produced and sold (not resold) in a specific time period by countries. Due to its complex and subjective nature this measure is ofte ...
for
GTP by
guanine nucleotide exchange factor
Guanine nucleotide exchange factors (GEFs) are proteins or protein domains that activate monomeric GTPases by stimulating the release of guanosine diphosphate (GDP) to allow binding of guanosine triphosphate (GTP). A variety of unrelated struc ...
s in the cell. Once GTP is bound to the myristoylated protein, it becomes activated, exposing the myristoyl group. These conformational switches can be utilized as a signal for cellular localization, membrane-protein, and
protein–protein interaction
Protein–protein interactions (PPIs) are physical contacts of high specificity established between two or more protein molecules as a result of biochemical events steered by interactions that include electrostatic forces, hydrogen bonding and th ...
s.
Dual modifications of myristoylated proteins
Further modifications on N-myristoylated proteins can add another level of regulation for myristoylated protein. Dual
acylation
In chemistry, acylation (or alkanoylation) is the chemical reaction in which an acyl group () is added to a compound. The compound providing the acyl group is called the acylating agent.
Because they form a strong electrophile when treated with ...
can facilitate more tightly regulated protein localization, specifically targeting proteins to
lipid rafts at membranes
or allowing dissociation of myristoylated proteins from membranes.
Myristoylation and
palmitoylation
Palmitoylation is the covalent attachment of fatty acids, such as palmitic acid, to cysteine (''S''-palmitoylation) and less frequently to serine and threonine (''O''-palmitoylation) residues of proteins, which are typically membrane protein ...
are commonly coupled modifications. Myristoylation alone can promote transient membrane interactions
that enable proteins to anchor to membranes but dissociate easily. Further palmitoylation allows for tighter anchoring and slower dissociation from membranes when required by the cell. This specific dual modification is important for
G protein-coupled receptor
G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily-related p ...
pathways and is referred to as the dual fatty acylation switch.
Myristoylation is often followed by
phosphorylation of nearby residues. Additional phosphorylation of the same protein can decrease the electrostatic affinity of the myristoylated protein for the membrane, causing
translocation of that protein to the cytoplasm following dissociation from the membrane.
Signal transduction
Myristoylation plays a vital role in membrane targeting and
signal transduction in plant responses to environmental stress. In addition, in signal transduction via G protein,
palmitoylation
Palmitoylation is the covalent attachment of fatty acids, such as palmitic acid, to cysteine (''S''-palmitoylation) and less frequently to serine and threonine (''O''-palmitoylation) residues of proteins, which are typically membrane protein ...
of the α subunit,
prenylation
Prenylation (also known as isoprenylation or lipidation) is the addition of hydrophobic molecules to a protein or a biomolecule. It is usually assumed that prenyl groups (3-methylbut-2-en-1-yl) facilitate attachment to cell membranes, similar to ...
of the γ subunit, and myristoylation is involved in tethering the G protein to the inner surface of the plasma membrane so that the G protein can interact with its receptor.
Apoptosis
Myristoylation is an integral part of
apoptosis, or programmed cell death. Apoptosis is necessary for cell homeostasis and occurs when cells are under stress such as
hypoxia or
DNA damage. Apoptosis can proceed by either mitochondrial or receptor mediated activation. In receptor mediated apoptosis, apoptotic pathways are triggered when the cell binds a death receptor. In one such case, death receptor binding initiates the formation of the
death-inducing signaling complex, a complex composed of numerous proteins including several caspases, including
caspase 3
Caspase-3 is a caspase protein that interacts with caspase-8 and caspase-9. It is encoded by the ''CASP3'' gene. ''CASP3'' orthologs have been identified in numerous mammals for which complete genome data are available. Unique orthologs are als ...
. Caspase 3 cleaves a number of proteins that are subsequently myristoylated by NMT. The
pro-apoptotic BH3-interacting domain death agonist (Bid) is one such protein that once myristoylated, translocates to the
mitochondria where it prompts the release of
cytochrome c leading to cell death.
Actin
Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ov ...
,
gelsolin and p21-activated kinase 2
PAK2
Serine/threonine-protein kinase PAK 2 is an enzyme that in humans is encoded by the ''PAK2'' gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''gen ...
are three other proteins that are myristoylated following cleavage by
caspase 3
Caspase-3 is a caspase protein that interacts with caspase-8 and caspase-9. It is encoded by the ''CASP3'' gene. ''CASP3'' orthologs have been identified in numerous mammals for which complete genome data are available. Unique orthologs are als ...
, which leads to either the up-regulation or down-regulation of apoptosis.
Impact on human health
Cancer
''c-Src'' is a gene that codes for proto-oncogene tyrosine-protein kinase Src, a protein important for normal
mitotic cycling. It is phosphorylated and dephosphorylated to turn signaling on and off. Proto-oncogene tyrosine-protein kinase Src must be localized to the
plasma membrane in order to phosphorylate other downstream targets; myristoylation is responsible for this
membrane targeting event. Increased myristoylation of ''c-Src'' can lead to enhanced
cell proliferation and be responsible for
transforming normal cells into cancer cells.
Activation of ''c-Src'' can lead to the so-called
hallmarks of cancer
The hallmarks of cancer were originally six biological capabilities acquired during the multistep development of human tumors and have since been increased to eight capabilities and two enabling capabilities. The idea was coined by Douglas Hanah ...
: upregulation of
angiogenesis, proliferation, and
invasion
An invasion is a Offensive (military), military offensive in which large numbers of combatants of one geopolitics, geopolitical Legal entity, entity aggressively enter territory (country subdivision), territory owned by another such entity, gen ...
.
Viral infectivity
HIV-1
The subtypes of HIV include two major types, HIV type 1 (HIV-1) and HIV type 2 (HIV-2). HIV-1 is related to viruses found in chimpanzees and gorillas living in western Africa, while HIV-2 viruses are related to viruses found in the sooty mangabey ...
is a
retrovirus that relies on myristoylation of one of its structural proteins in order to successfully package its genome, assemble and mature into a new infectious particle.
Viral matrix protein
Viral matrix proteins are structural proteins linking the viral envelope with the virus core. They play a crucial role in virus assembly, and interact with the RNP complex as well as with the viral membrane. They are found in many enveloped virus ...
, the N-terminal most domain of the
gag polyprotein is myristoylated.
This myristoylation modification targets gag to the membrane of the host cell. Utilizing the myristoyl-electrostatic switch,
including a basic patch on the matrix protein,
gag
A gag is usually an item or device designed to prevent speech, often as a restraint device to stop the subject from calling for help and keep its wearer silent. This is usually done by blocking the mouth, partially or completely, or attemptin ...
can assemble at
lipid raft
The plasma membranes of cells contain combinations of glycosphingolipids, cholesterol and protein receptors organised in glycolipoprotein lipid microdomains termed lipid rafts. Their existence in cellular membranes remains somewhat controversial ...
s at the
plasma membrane for
viral assembly, budding and further maturation.
In order to prevent viral infectivity, myristoylation of the matrix protein could become a good drug target.
Prokaryotic and eukaryotic infections
Certain NMTs are therapeutic targets for development of drugs against bacterial
infection
An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dis ...
s. Myristoylation has been shown to be necessary for the survival of a number of disease-causing
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
, among them ''
C. albicans'' and ''
C. neoformans
''Cryptococcus neoformans'' is an encapsulated yeast belonging to the class (biology), class Tremellomycetes and an obligate aerobe that can live in both plants and animals. Its teleomorph is a filamentous fungus, formerly referred to ''Filobasid ...
''. In addition to
prokaryotic
A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...
bacteria, the NMTs of numerous disease-causing
eukaryotic
Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
organisms have been identified as
drug target
A biological target is anything within a living organism to which some other entity (like an endogenous ligand or a drug) is directed and/or binds, resulting in a change in its behavior or function. Examples of common classes of biological targets ...
s as well. Proper NMT functioning in the
protozoa ''
Leishmania major'' and ''
Leishmania donovani
''Leishmania donovani'' is a species of intracellular parasites belonging to the genus ''Leishmania'', a group of haemoflagellate kinetoplastids that cause the disease leishmaniasis. It is a human blood parasite responsible for visceral leishmani ...
'' (
leishmaniasis), ''
Trypanosoma brucei'' (
African sleeping sickness), and ''
P. falciparum'' (
malaria
Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms that typically include fever, tiredness, vomiting, and headaches. In severe cases, it can cause jaundice, seizures, coma, or death. S ...
) is necessary for survival of the parasites. Inhibitors of these organisms are under current investigation. A
pyrazole
Pyrazole is an organic compound with the formula C3H3N2H. It is a heterocycle characterized by a 5-membered ring of three carbon atoms and two adjacent nitrogen atoms, which are in ortho-substitution. Pyrazole is a weak base, with p''K''b 11.5 ( ...
sulfonamide inhibitor has been identified that selectively binds ''T. brucei'', competing for the
peptide binding site, thus inhibiting enzymatic activity and eliminating the parasite from the bloodstream of mice with
African sleeping sickness.
See also
*
Acylation
In chemistry, acylation (or alkanoylation) is the chemical reaction in which an acyl group () is added to a compound. The compound providing the acyl group is called the acylating agent.
Because they form a strong electrophile when treated with ...
*
Prenylation
Prenylation (also known as isoprenylation or lipidation) is the addition of hydrophobic molecules to a protein or a biomolecule. It is usually assumed that prenyl groups (3-methylbut-2-en-1-yl) facilitate attachment to cell membranes, similar to ...
*
Palmitoylation
Palmitoylation is the covalent attachment of fatty acids, such as palmitic acid, to cysteine (''S''-palmitoylation) and less frequently to serine and threonine (''O''-palmitoylation) residues of proteins, which are typically membrane protein ...
*
Palmitoleoylation
*
Glycophosphatidylinositol
Glycosylphosphatidylinositol (), or glycophosphatidylinositol, or GPI in short, is a phosphoglyceride that can be attached to the C-terminus of a protein during posttranslational modification. The resulting GPI-anchored proteins play key roles i ...
References
External links
The MYR PredictorExPASy Myristoylation Tool
{{Protein posttranslational modification
Post-translational modification
Signal transduction