Sterol Regulatory Element-binding Protein - Mechanism of Action

Mechanism of Action

Animal cells maintain proper levels of intracellular lipids (fats and oils) under widely varying circumstances (lipid homeostasis). 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.

A 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 the cholesterol-sensing protein SREBP cleavage activating protein (Scap), which forms a complex with SREBP owing to interaction between their respective carboxy-terminal domains. Scap, in turn, can bind reversibly with another ER-resident membrane protein, Insig. In the presence of sterols, which bind to Insig and Scap, Insig and Scap also bind one another. Insig always stays in the ER membrane and thus the SREBP:Scap complex remains in the ER when Scap is bound to Insig. When sterol levels are low, Insig and Scap no longer bind. Then, Scap undergoes a conformational change that exposes a portion of the protein ('MELADL') that signals it to be included as cargo in the COPII vesicles that move from the ER to the Golgi apparatus. In these vesicles, Scap, dragging SREBP along with it, is transported to the Golgi. The 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.

Once in the Golgi apparatus, 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 (e.g. LDL receptor gene)