Sterol regulatory element binding protein

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Sterol Regulatory Element Binding Proteins (SREBPs) are transcription factors that bind to the sterol regulatory element DNA sequence TCACNCCAC. SREBPs belong to the basic-helix-loop-helix leucine zipper class of transcription factors.^[2] Unactivated SREBPs are attached to the nuclear envelope and endoplasmic reticulum membranes. In cells with low levels of sterols, SREBPs are cleaved to a water soluble N-terminal domain which is translocated to the nucleus. These activated SREBPs then bind to specific sterol regulatory element DNA sequences which upregulate the synthesis of enzymes involved in sterol biosynthesis.^[3][4] Sterols in turn inhibit the cleavage of SREBPs and therefore synthesis of additional sterols is reduced through a negative feed back loop.

Mechanism of action

Beginning with the discovery of the Sterol Regulatory Element Binding Proteins (SREBPs) in 1993,^[2] a productive combination of biochemistry, molecular biology and genetics, has brought to light the complex mechanisms by which animal cells maintain the proper levels of intracellular lipid (fats and oils) in the face of widely varying circumstances (lipid homeostasis).^[1][5][6] These studies exposed a signaling mechanism of beguiling complexity that is responsible for the end-product feedback regulation of gene transcription. For example, when cellular cholesterol levels fall below the level needed, the cell makes more of the enzymes necessary to make cholesterol. A principal step in this response is to make more of the mRNA transcripts that direct the synthesis of these enzymes. Conversely, when there is enough cholesterol around, the cell stops making those mRNAs and the level of the enzymes falls. As a result, the cell quits making cholesterol once it has enough.

A notable feature of this regulatory feedback machinery was first observed for the SREBP pathway - regulated intramembrane proteolysis (Rip). Subsequently, Rip was found to be used in almost all organisms from bacteria to human beings and regulates a wide range of processes ranging from development to neurodegeneration.

The defining feature of the SREBP pathway is the proteolytic release of a membrane-bound transcription factor, SREBP. Proteolytic cleavage frees it to move through the cytoplasm to the nucleus. Once in the nucleus, SREBP can bind to specific DNA sequences (the sterol regulatory elements or SREs) that are found in the control regions of the genes that encode enzymes needed to make lipids. This binding to DNA leads to the increased transcription of the target genes.

The ~120 kDa SREBP precursor protein is anchored in the membranes of the endoplasmic reticulum (ER) and nuclear envelope by virtue of two membrane-spanning helices in the middle of the protein. The precursor has a hairpin orientation in the membrane, so that both the amino-terminal transcription factor domain and the COOH-terminal regulatory domain face the cytoplasm. The two membrane-spanning helices are separated by a loop of about 30 amino acids that lies in the lumen of the ER. Two separate, site-specific proteolytic cleavages are necessary for release of the transcriptionally active amino-terminal domain. These cleavages are carried out by two distinct proteases, called site-1 protease (S1P) and site-2 protease (S2P).

In addition to S1P and S2P, the regulated release of transcriptionally active SREBP requires SREBP cleavage activating protein (SCAP), which forms a complex with SREBP owing to interaction between their respective carboxy-terminal domains.

Regulation of SREBP cleavage employs a notable feature of eukaryotic cells, subcellular compartmentalization defined by intracellular membranes, to ensure that cleavage occurs only when needed.

Feedback regulation of SREBP processing is best understood in the case of cholesterol. When cellular demand for cholesterol rises, the SREBP:SCAP complex exits the ER and travels to the Golgi apparatus where the SREBP:SCAP complex encounters active S1P. S1P cleaves SREBP at site-1, cutting it into two halves. Because each half still has a membrane-spanning helix, each remains bound in the membrane. The newly generated amino-terminal half of SREBP (which is the ‘business end' of the molecule) then goes on to be cleaved at site-2 that lies within its membrane-spanning helix. This is the work of S2P, an unusual metalloprotease. This releases the cytoplasmic portion of SREBP, which then travels to the nucleus where it activates transcription of target genes.

Isoforms

Mammalian genomes have two separate SREBP genes (SREBF1 and SREBF2):

• SREBP-1 expression produces two different isoforms, SREBP-1a and -1c. These isoforms differ in their first exons owing to the use of different transcriptional start sites for the SREBP-1 gene. SREBP-1c was also identified in rats as ADD-1. REBP-1 is responsible for regulating the genes needed to make fatty acids.
• SREBP-2. SREBP-2 regulates the genes of cholesterol metabolism.

References