Collagen type IV alpha-3-binding protein, also known as ceramide transfer protein (CERT) or StAR-related lipid transfer protein 11 (STARD11) is a

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

that in humans is encoded by the ''COL4A3BP''

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

. The protein contains a

pleckstrin homology domain Pleckstrin homology domain (PH domain) or (PHIP) is a protein domain of approximately 120 amino acids that occurs in a wide range of proteins involved in intracellular signaling or as constituents of the cytoskeleton. This domain can bind phosph ...

at its amino terminus and a

START domain START (StAR-related lipid-transfer) is a lipid-binding domain in StAR, HD-ZIP and signalling proteins. The archetypical domain is found in StAR (Steroidogenic acute regulatory protein), a mitochondrial protein that is synthesized in steroid-pro ...

towards the end of the molecule. It is a member of the StarD2 subfamily of START domain proteins.

Function and structure

Ceramide transferase protein (or CERT) is responsible for the transfer of

ceramide Ceramides are a family of waxy lipid molecules. A ceramide is composed of sphingosine and a fatty acid joined by an amide bond. Ceramides are found in high concentrations within the cell membrane of Eukaryote, eukaryotic cells, since they are co ...

from the

endoplasmic reticulum The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryote, eukaryotic cell, and has many other important functions such as protein folding. The word endoplasmic means "within the cytoplasm", and reticulum is Latin for ...

(ER) to the

Golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic Cell (biology), cells. Part of the endomembrane system in the cytoplasm, it protein targeting, packages proteins ...

. Ceramide plays a very important role in the metabolism and biosynthesis of

sphingolipid Sphingolipids are a class of lipids containing a backbone of sphingoid bases, which are a set of aliphatic amino alcohols that includes sphingosine. They were discovered in brain extracts in the 1870s and were named after the mythological sp ...

. More specifically, it is synthesized at the ER, then is transferred by CERT to Golgi where it is converted to

sphingomyelin Sphingomyelin (SPH, ) is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath that surrounds some nerve cell axons. It usually consists of phosphocholine and ceramide, or a phosphoethanolamine hea ...

(SM). There are two pathways through which this transfer takes place: a major pathway, which is ATP and cytosol-dependent and a minor pathway, which is ATP- and cytosol-independent. CERT is a 68kDa protein that consists of three different parts, each of which with a special role: #

Pleckstrin homology domain Pleckstrin homology domain (PH domain) or (PHIP) is a protein domain of approximately 120 amino acids that occurs in a wide range of proteins involved in intracellular signaling or as constituents of the cytoskeleton. This domain can bind phosph ...

(PH): It is the aminoterminal domain and it consists of about 100 aminoacid residues. The main function of this part of CERT is to recognize and bind various phosphatidylinositol phosphates (PIPs) with different level of specificity. The isomers of PIPs are distributed to various organelles: PI-4,5-diphosphate goes to the plasma membrane, PI-3-monophosphate to endosomes and PI-4-monophosphate to Golgi. PH domain of wild-type CERT has been found to recognize specifically PI4P and therefore CERT targets the Golgi apparatus or the trans-Golgi network. #

: It consists of about 210 amino acid residues and has an important role in the transfer of ceramide, which is that it can recognize specifically only the natural D-erythro isomer of ceramide and extract it from the membrane. #

FFAT motif A FFAT motif (FFAT being an acronym for two phenylalanines (FF) in an acidic tract) is a protein sequence motif of six defined amino acids plus neighbouring residues that binds to proteins in the VAP protein family. Initial definition The classic ...

(two

phenylalanine Phenylalanine (symbol Phe or F) is an essential α-amino acid with the chemical formula, formula . It can be viewed as a benzyl group substituent, substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of ...

s in an acidic tract, that has a conserved sequence "EFFDAxE"): It is a short domain situated between PH and START domain and is the one responsible for the interaction of CERT with ER. More specifically, it binds to the ER resident type II membrane protein,

vesicle-associated membrane protein Vesicle associated membrane proteins (VAMPs) are a family of SNARE proteins with similar structure, and are mostly involved in vesicle fusion. * VAMP1 and VAMP2 proteins known as synaptobrevins are expressed in brain and are constituents o ...

(VAMP) associated protein (VAP), an interaction that is necessary for the transfer of ceramide from the ER to Golgi. All of these domains are important for the transfer of ceramide, since first of all CERT will extract newly synthesized ceramide from the membrane with the help of its START domain. Then, ceramide will be transferred through the cytosol towards Golgi because of the interaction between the PH domain and PI4P. Finally, interaction with ER is facilitated through the binding of the FFAT motif with vesicle-associated membrane protein.

Regulation

The transport of ceramide by CERT requires ATP. CERT – when expressed in mammalian cells – has been found to receive a lot of possible phosphorylations at the serine repeat (SR) motif, which is close to the PH domain. It has been shown that the

phosphorylation In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writ ...

of this SR motif leads to inactivation of the PI4P-binding and ceramide transferring activities of CERT, since it induces an autoinhibitory reaction between the PH and START domains of CERT, transforming it from the active form to the inactive form.

Protein kinase D 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, respon ...

(PKD) has been found to phosphorylate the SR motif of CERT. Also, CERT is further phosphorylated by the casein kinase 1 family leading to hyperphosphorylation of the SR motif. On the other hand, the integral membrane protein protein phosphatase 2Cε (PP2Cε), which is located on the endoplasmic reticulum induces dephosphorylation of CERT. Dephosphorylated CERT is in the active form in order to be functional and transfer ceramide from ER to Golgi.

Inhibitor HPA-12

The chemically synthesized compound N-(30hydroxy-1-hydroxymethyl-3-phenylpropyl)dodecamide (HPA-12) has been found to be an inhibitor of CERT-mediated ceramide trafficking. More specifically, this drug inhibits the ATP-dependent transport of ceramide from ER to Golgi (and therefore the conversion of ceramide to sphingomyelin), but it does not inhibit protein trafficking. This suggests that Ceramide is still transformed to Glycosylceramide at Golgi. Moreover, it has been shown that it does not inhibit the Sphingomyelin synthase in vitro or in vivo. Moreover, only the (1R, 3R) isomer of HPA-12 has been found to be an active inhibitor and the length of the chain as well as the two hydroxyl-groups are very important for the inhibitory activity.

Clinical significance

This gene encodes a kinase also known as Goodpasture antigen-binding protein that specifically phosphorylates the

N-terminal The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amin ...

region of the non-collagenous domain of the alpha 3 chain of type IV collagen, known as the Goodpasture

antigen In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...

. Goodpasture's syndrome is the result of an

autoimmune In immunology, autoimmunity is the system of immune responses of an organism against its own healthy cells, tissues and other normal body constituents. Any disease resulting from this type of immune response is termed an " autoimmune disease" ...

response directed at this antigen. One isoform of this protein is also involved in ceramide intracellular transport. Two transcripts exist for this gene.

References

External links