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Section 6 - Membrane Proteins.docx

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Biology 2382B
Cumming/ Damjanovski

Section VI: Membrane Proteins The exoplasmic side of the membrane can be called different things – luminar, extracellular. Integral proteins • Always in the same orientation, i.e. cadherins • Must have a transmembrane domain – must have hydrophobic components. Must span plasma membrane. 2 structures formed as a transmembrane domain – beta barrel and more commonly a-helix: simple helical structure that spans the transmembrane domain. A-helix – transmembrane domain is 20-25 amino acids • Also needs cytoplasmic and exoplasmic domain – the sizes can vary. Ideally part is extracellular and one is cytoplasmic. • Hydrophilic (charged) residues Charged amino acids in the cytosolic side of most transmembrane proteins. Function: If you try to pull the protein from the extracellular side, the charged aa will resist being pulled in the transmembrane region. This anchors the protein there. • Exoplasmic domain can be glycosolated (sugars can be added) i.e. Found in the lumen of ER and golgi. The proteins that are modified can be glycosylated because they are in the exoplasmic section. Lipid Linked Proteins Proteins are linked to existing phosophates in the phospholipids in the bilayer. Proteins are linked to lipids through 3 linkages: • Glycosulphosphatidulinositol (GPI) anchors. Need phosphotydulinositol (a phosphoglyceride – type of phospholipid) to link a protein to the GPI anchor. GPI anchors are always on the external leaflet (exoplasmic face). Certain proteins can be linked to the phosphotydulinositol. GPI anchors link proteins to the external surface through phosphotid… Some N-cams can be integral membrane proteins: extracellular, cytoplasmic and transmembrane anchored. When integral they can link cytoskeleton and be involved in signaling. if N-Cam is GPI anchored, it only has an extracellular domain. It can still be involved in cell adhesion but it does not have a cytosolic domain, cannot link to cytoskeleton, not involved in signaling. • Acylation: Adding proteins to the cytosolic side of the membrane and needs N terminal Glycine. If a protein has an N-terminal glycine, it can be linked to the cytosolic side of the membrane. • Prenylation: adding proteins to the cytosolic side of the membrane needs a C-terminal Cysteine or near the C-terminal. Lipid linked proteins are attached covalently so they are part of the plasma membrane. Peripheral Proteins • Are bound non-covalently to other proteins that are already bound to the bilayer (either lipid- linked or integral membrane proteins). Bound non-covalently through van der walls interactions or ionically. i.e. dystrophin is bound to the cytoskeleton through a transmembrane protein. • Can be a peripheral membrane protein on either extracellular or cytoplasmic side. • ECM can be a peripheral membrane protein but not always. i.e. if it is bound to an integrin (transmembrane protein) than it is a peripheral membrane protein ECM is not a peripheral membrane protein if it is not bound to either lipid linked or transmembrane. Insertion of Proteins into Membranes • GPI is not part of this it is a totally different process • Putting proteins into the membrane and try to understand where is the N terminal domain and where the C terminal domain I and how many transmembrane domains are going to be in this protein • Depending on where the N and C terminal domains are, and how many transmembrane domains total, the integral proteins can be different types. We should know which type results in what orientation. Signal Sequences All translation is occurring in the cytosol. Ribosome binds to RNA and translation of protein starts in cytosol. In order to get protein to a membrane needs a sequence. • The N terminal sequence is a topogenic sequence – they are in a certain shape because of the hydrophobic sequences folding. The shape is recognized. Types of signal sequences: • N terminal signal sequence • Signal anchor sequence The word “signal” means to go to the ER • Stop-transfer/membrane anchor sequence Stop transferring to ER and anchor the protein here. • Tail anchored proteins have topogenic sequence in C-terminus Hydrophobic C-terminal domain is the topogenic sequence Pattern of topogenic sequences is what results in the different types of membrane protein. Tail Anchored proteins Translation occurs starting from the N-terminal domain and encounters a topogenic sequence in the hydrophobic C-terminal domain. Translation starts in the cytoplasm. This is a special case, not many proteins do this. • Once there is translation of this protein, it has a C-terminus hydrophobic sequence. The hydrophobic tail is in the cytosol and forms a topogenic shape recognized by Get3. • Get3 recognizes the sequence that means to shove it into the membrane of the ER. • Get3 with the help of ATP, Get1 and Get2 complex shove the C terminus of the protein into the membrane of the ER. The N terminus remains in the cytoplasm. • Tail anchored proteins always end up with the N terminal domain in the cytosol (cytosolic domain) and the C-terminal domain in the membrane (transmembrane domain). • Does not have a lumenal or extracellular domain. • Example: Snare proteins Type I proteins • Have N terminal signal sequence and stop transfer membrane anchor sequence. N terminal sequence is the first translated so right away it signals to be taken to the ER. In Type I proteins, N-terminal domain always found in the lumen. Once the nascent protein gets to the ER, N terminal sequence get cleaved off after it has gone through the translocon. Strop transfer anchor sequence – translation continues into the cytoplasm. Stop-transfer sequence is anchored into the membrane and becomes the transmembrane domain. The rest of the protein is translated in the cytosol. N terminal domain is always luminal and C terminal domain is cytoplasmic. NOTE: Whether it is a secretory protein or a Type I transmembrane protein, the N terminal sequence is cleaved. If there is no stop transfer, then the protein is secreted. Type II and Type III • Internal Signal anchor-sequence is going to be the transmembrane domain. That signal sequence says take me to the ER. • The N-terminal domain has already been made but once you hit this sequence, that is when the cell realizes that it needs to be moved. • (Type II) If there is positive charges around the signal anchor sequence are on the N terminal side, than the nascent protein gets rotated so that the N-terminus will be in the cytosol and the C terminal domain will be translated into the lumen. • (Type II) If there are no charges on the N terminus side, then it continues being translated through to the exoplasmic side. On the C-terminal side of the sequence, there are positive charges, so the C terminus side is going to be in the cytosolic side. • After translation is completed, the internal sequence leads the protein into the lipid bilayer. • There are other charged amino acids in other parts of the proteins but the ones next to the internal sequence are special. Type IV • Multiple transmembrane domains. • Varies on whether the N or C terminus is luminal or exoplasmic. •
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