Perilipin, also known as lipid droplet-associated protein, Perilipin 1, or PLIN, is a
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
that, in humans, is encoded by the ''PLIN''
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
.
The perilipins are a family of proteins that associate with the surface of
lipid droplet
Lipid droplets, also referred to as lipid bodies, oil bodies or adiposomes, are lipid-rich cellular organelles that regulate the storage and hydrolysis of neutral lipids and are found largely in the adipose tissue. They also serve as a reservoi ...
s. Phosphorylation of perilipin is essential for the mobilization of fats in adipose tissue.
[Mobilization and Cellular Uptake of Stored Fats (with Animation)](_blank)
/ref>
Perilipin family of proteins
Perilipin is part of a gene family with six currently-known members. In vertebrates
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
, closely related genes include adipophilin
Adipose differentiation-related protein, also known as perilipin 2 , ADRP or adipophilin, is a protein which belongs from PAT family of cytoplasmic lipid droplet(CLD) binding protein. In humans it is encoded by the ''ADFP'' gene. This protein surr ...
(also known as adipose differentiation-related protein or ''Perilipin 2''), TIP47
Mannose-6-phosphate receptor binding protein 1 (M6PRBP1) is a protein which in humans is encoded by the ''M6PRBP1'' gene. Its gene product, as well as the gene itself, is commonly known as TIP47.
Function
Mannose 6-phosphate receptors (MPRs) ...
(''Perilipin 3''), ''Perilipin 4'' and ''Perilipin 5
Perilipin 5, also known as Oxpatperilipin 5 or PLIN5, is a protein that belongs to perilipin family. This protein group has been shown to be responsible for lipid droplet's biogenesis, structure and degradation. In particular, Perilipin 5 is a li ...
'' (also called MLDP, LSDP5, or OXPAT). Insects express related proteins, LSD1
Lysine-specific histone demethylase 1A (LSD1) also known as lysine (K)-specific demethylase 1A (KDM1A) is a protein in humans that is encoded by the KDM1A gene. LSD1 is a flavin-dependent monoamine oxidase, which can demethylate mono- and di-me ...
and LSD2, in fat bodies. The yeast ''Saccharomyces cerevisiae'' expresses PLN1 (formerly PET10), that stabilizes lipid droplets and aids in their assembly.
Evolution
The perilipins are considered to have their origins in a common ancestral gene which, during the first and second vertebrate genome duplication, Â gave rise to six types of PLIN genes.
Composition and structure
Human Perilipin
Human perilipin-1 is composed by 522 amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s, which add up to a molecular mass of 55.990 kDa. It presents an estimated number of 15 phosphorylation sites (residues 81, 85, 126, 130, 132, 137, 174, 299, 301, 382, 384, 408, 436, 497, 499 and 522) from which 3 -those in bold- have been suggested to be relevant for stimulated-lipolysis through PKA phosphorylation - they correspond respectively to PKA Phosphorylation sites 1, 5 and 6. A compositional bias of Glutamic acid
Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can synt ...
can be found between residues 307 and 316. Its secondary structure
Protein secondary structure is the three dimensional conformational isomerism, form of ''local segments'' of proteins. The two most common Protein structure#Secondary structure, secondary structural elements are alpha helix, alpha helices and beta ...
has been suggested to be conformed exclusively by partially hydrophobic α-helixes, as well as the respective coils and bends.
Whereas perilipin-1 is coded by a single gene, alternative mRNA splicing
RNA splicing is a process in molecular biology where a newly-made precursor messenger RNA (pre-mRNA) transcript is transformed into a mature messenger RNA (mRNA). It works by removing all the introns (non-coding regions of RNA) and ''splicing'' ba ...
processes can led to three protein isoform
A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene or gene family and are the result of genetic differences. While many perform the same or similar biological roles, some isof ...
s (Perilipin A, B and C). Both Perilipin A and B present common N-terminal regions, differing in the C-terminal ones. Concretely, beginning from the N-terminal of Perilipin-1, a PAT domain -characteristic of its protein family- can be found, followed by also characteristic repeated sequence of 13 residues -which form amphipathic
An amphiphile (from the Greek αμφις amphis, both, and φιλÃα philia, love, friendship), or amphipath, is a chemical compound possessing both hydrophilic (''water-loving'', polar) and lipophilic (''fat-loving'') properties. Such a compoun ...
helixes with an active role in linking membranes.- and a 4-helix bundle before the C-terminal carbon In Perilipin A, lipophile
Lipophilicity (from Greek λίπος "fat" and φίλος "friendly"), refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. Such non-polar solvents are themselves lipop ...
nature is conferred by the slightly hydrophobic amino acids concentrated in the central 25% of the sequence
In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called ''elements'', or ''terms''). The number of elements (possibly infinite) is calle ...
, region that anchors the protein to the core of the lipid droplet.
Murine Perilipin
Unlike its human orthologous it is composed by 517 amino acids in the primary structure of which several regions can be identified. Three moderately 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, th ...
sequences (H1, H2, H3) of 18 rem (243-260 aa), 23 rem (320-332 aa) and 16 rem (349-364 aa) can be identified in the centre of the protein, as well as an acidic region of 28 residues where both glutamic and aspartic
Aspartic acid (symbol Asp or D; the ionic form is known as aspartate), is an α-amino acid that is used in the biosynthesis of proteins. Like all other amino acids, it contains an amino group and a carboxylic acid. Its α-amino group is in the pro ...
acids add up to 19 of them. Five sequences 18 residues long that could form amphipathic β-pleated sheets -according to a prediction made through LOCATE program- are found between aa 111 and 182. Serine
Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...
s occupying positions 81, 222, 276, 433, 492 and 517 act as phosphorylation sites -numbered from 1 to 6- for PKA, as well as several other threonine
Threonine (symbol Thr or T) is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), a carboxyl group (which is in the deprotonated −COOâ ...
s and serine
Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...
s which add up to 27 phosphorylation sites.
Function
Perilipin is a protein that coats lipid droplet
Lipid droplets, also referred to as lipid bodies, oil bodies or adiposomes, are lipid-rich cellular organelles that regulate the storage and hydrolysis of neutral lipids and are found largely in the adipose tissue. They also serve as a reservoi ...
s (LDs) in adipocyte
Adipocytes, also known as lipocytes and fat cells, are the cells that primarily compose adipose tissue, specialized in storing energy as fat. Adipocytes are derived from mesenchymal stem cells which give rise to adipocytes through adipogenesis. I ...
s, the fat
In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food.
The term often refers spec ...
-storing cells in adipose tissue
Adipose tissue, body fat, or simply fat is a loose connective tissue composed mostly of adipocytes. In addition to adipocytes, adipose tissue contains the stromal vascular fraction (SVF) of cells including preadipocytes, fibroblasts, vascular e ...
. In fact, PLIN1 is greatly expressed in white adipocytes.
It controls adipocyte lipid metabolism
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
. It handles essential functions in the regulation of basal and hormonally stimulated lipolysis
Lipolysis is the metabolic pathway through which lipid triglycerides are hydrolyzed into a glycerol and free fatty acids. It is used to mobilize stored energy during fasting or exercise, and usually occurs in fat adipocytes. The most important ...
and also rises the formation of large LDs which implies an increase in the synthesis of triglyceride
A triglyceride (TG, triacylglycerol, TAG, or triacylglyceride) is an ester derived from glycerol and three fatty acids (from ''tri-'' and ''glyceride'').
Triglycerides are the main constituents of body fat in humans and other vertebrates, as w ...
s.
In humans, Perilipin A is the most abundant protein associated with the adipocyte LDs and lower PLIN1 expression is related with higher rates of lipolysis.
Under basal conditions, Perilipin acts as a protective coating of LDs from the body's natural lipase
Lipase ( ) is a family of enzymes that catalyzes the hydrolysis of fats. Some lipases display broad substrate scope including esters of cholesterol, phospholipids, and of lipid-soluble vitamins and sphingomyelinases; however, these are usually tr ...
s, such as hormone-sensitive lipase
Hormone-sensitive lipase (, HSL), also previously known as cholesteryl ester hydrolase (CEH), sometimes referred to as triacylglycerol lipase, is an enzyme that, in humans, is encoded by the ''LIPE'' gene, and catalyzes the following reaction:
...
(HSL) and adipose triglyceride lipase
Adipose triglyceride lipase, also known as patatin-like phospholipase domain-containing protein 2 and ATGL, is an enzyme that in humans is encoded by the ''PNPLA2'' gene. ATGL catalyses the first reaction of lipolysis, where triacylglycerols are ...
(ATGL), which break triglycerides into glycerol
Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...
and free fatty acid
In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, fr ...
s for use in lipid metabolism.
In times of energy deficit, Perilipin is hyperphosphorylated
Hyperphosphorylation occurs when a biochemical with multiple phosphorylation sites is fully saturated. Hyperphosphorylation is one of the signaling mechanisms used by the cell to regulate mitosis. When these mechanisms fail, developmental problem ...
by PKA following β-adrenergic
The adrenergic receptors or adrenoceptors are a class of G protein-coupled receptors that are targets of many catecholamines like norepinephrine (noradrenaline) and epinephrine (adrenaline) produced by the body, but also many medications like beta ...
receptor activation. Phosphorylated perilipin changes conformation, exposing the stored lipids to hormone-sensitive lipase-mediated lipolysis.
Modulator of adipocyte lipid metabolism
Specifically, in the basal state Perilipin A allows a low level of basal lipolysis by reducing the access of cytosolic lipases to stored triacylglycerol in LDs. It is found at their surface in a complex with CGI-58, the co-activator of ATGL. ATGL might also be in this complex but it is quiescent.
Under lipolytically stimulated conditions, PKA is activated and phosphorylates up to 6 Serine
Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...
residues on Perilipin A (Ser81, 222, 276, 433, 492, and 517) and 2 on HSL (Ser659, and 660). Although PKA also phosphorylates HSL, which can increase its activity, the more than 50-fold increase in fat mobilization (triggered by epinephrine
Adrenaline, also known as epinephrine, is a hormone and medication which is involved in regulating visceral functions (e.g., respiration). It appears as a white microcrystalline granule. Adrenaline is normally produced by the adrenal glands and ...
) is primarily due to Perilipin phosphorylation.
Then, Phosphorylated HSL translocates to the LD surface and associates with Perilipin A and Adipocyte fatty acid-binding protein (AFABP). Consequently, HSL gains access to triacylglycerol (TAG) and diacylglycerol (DAG), substrates in LDs. Also, CGI-58 separates from the LD outer layer which leads to a redistribution of ATGL. In particular, ATGL interacts with Perilipin A through phosphorylated Ser517.
As a result, PKA phosphorylation implies an enriched colocation of HLS and ATGL which facilitates maximal lipolysis by the two lipases.
Clinical significance
Perilipin is an important regulator of lipid storage. Both an overexpression or deficiency of the protein, caused by a mutation, lead to severe health issues.
Overexpression
Perilipin expression
Expression may refer to:
Linguistics
* Expression (linguistics), a word, phrase, or sentence
* Fixed expression, a form of words with a specific meaning
* Idiom, a type of fixed expression
* Metaphorical expression, a particular word, phrase, o ...
is elevated in obese
Obesity is a medical condition, sometimes considered a disease, in which excess body fat has accumulated to such an extent that it may negatively affect health. People are classified as obese when their body mass index (BMI)—a person's we ...
animals and humans. Polymorphisms in the human perilipin (PLIN) gene have been associated with variance in body-weight regulation and may be a genetic influence on obesity risk in humans.
This protein can me modified by O-linked acetylglucosamine ( O-GlNac) moieties and the enzyme that intervenes is O-GlcNAc transferase
Protein ''O''-GlcNAc transferase also known as OGT or O-linked N-acetylglucosaminyltransferase is an enzyme () that in humans is encoded by the ''OGT'' gene. OGT catalyzes the addition of the ''O''-GlcNAc post-translational modification to prote ...
(OGT). An abundance of OGT obstructs lipolysis
Lipolysis is the metabolic pathway through which lipid triglycerides are hydrolyzed into a glycerol and free fatty acids. It is used to mobilize stored energy during fasting or exercise, and usually occurs in fat adipocytes. The most important ...
and benefits diet-induced obesity and whole-body insulin resistance. Studies also propose that an overexpression of adipose O-GlcNAc signaling is a molecular expression of obesity and diabetes in humans.
Deficiency
Perilipin-null mice eat more food than wild-type
The wild type (WT) is the phenotype of the typical form of a species as it occurs in nature. Originally, the wild type was conceptualized as a product of the standard "normal" allele at a locus, in contrast to that produced by a non-standard, "m ...
mice, but gain 1/3 less fat than wild-type mice on the same diet; perilipin-null mice are thinner, with more lean muscle mass.['']telegraph.co.uk
''The Daily Telegraph'', known online and elsewhere as ''The Telegraph'', is a national British daily broadsheet newspaper published in London by Telegraph Media Group and distributed across the United Kingdom and internationally.
It was fo ...
'', 19 June 2001, Perilipin-null mice also exhibit enhanced leptin
Leptin (from Ancient Greek, Greek λεπτός ''leptos'', "thin" or "light" or "small") is a hormone predominantly made by adipose cells and enterocytes in the small intestine that helps to regulate Energy homeostasis, energy balance by inhib ...
production and a greater tendency to develop insulin resistance
Insulin resistance (IR) is a pathological condition in which cell (biology), cells fail to respond normally to the hormone insulin.
Insulin is a hormone that facilitates the transport of glucose from blood into cells, thereby reducing blood gluco ...
than wild-type mice. Even though perilipin-null mice present less fat mass and a higher insulin resistance, they do not show signs of a whole lipodystrophic phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
.
In humans, studies suggest that a deficiency of PLIN1 causes lipodystrophic syndromes, which disables the optimal accumulation of triglycerides in adipocytes that derives in an abnormal deposition of lipids in tissues such as skeletal muscle and liver. The storage of lipids in the liver leads to insulin resistance and hypertriglyceridemia
Hypertriglyceridemia is the presence of high amounts of triglycerides in the blood. Triglycerides are the most abundant fatty molecule in most organisms. Hypertriglyceridemia occurs in various physiologic conditions and in various diseases, and h ...
. Affected patients are characterized by a subcutaneous fat with smaller than normal adipocytes, macrophage infiltration and fibrosis
Fibrosis, also known as fibrotic scarring, is a pathological wound healing in which connective tissue replaces normal parenchymal tissue to the extent that it goes unchecked, leading to considerable tissue remodelling and the formation of perma ...
.
These findings affirm a new primary form of inherited lipodystrophy
Lipodystrophy syndromes are a group of genetic or acquired disorders in which the body is unable to produce and maintain healthy fat tissue. The medical condition is characterized by abnormal or degenerative conditions of the body's adipose tissue. ...
and emphasize on the severe metabolic consequences of a defect in the formation of lipid droplets in adipose tissue.
In particular, variants 13041A>G and 14995A>T have been associated with increased risk of obesity in women and 11482G>A has been associated with decreased perilipin expression and increased lipolysis in women.
References
Further reading
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Metabolism
Human proteins