Biology 2382B Lecture Notes - Isoprene, Partition Coefficient, Translocon

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Published on 17 Apr 2013
School
Western University
Department
Biology
Course
Biology 2382B
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Section 6: Membrane Proteins
Three types of membrane
proteins:
1. Integral
2. Lipid-linked
3. Peripheral
Integral Membrane Proteins
- also called
transmembrane
proteins, span a
phospholipid bilayer and comprise three segments
- the cytosolic and exoplasmic domains have hydrophilic exterior
surfaces that interact with the aqueous solutions on the cytosolic
and exoplasmic domains
- the segments resemble water-soluble proteins in their amino acid
composition and structure
- the transmembrane domains consist of one or more alpha helices or
of multiple beta strands
α-Helices
- alpha helices are approximately 20-25 amino acids, Arg and Lys
(charged AA’s) near cytosolic side interact with polar head groups
- a single alpha domain is sufficient to incorporate an integral
membrane protein into a membrane, however many proteins have more than one alpha helix
domain
- The predicted length of such an alpha helix is just sufficient to span the hydrocarbon core of the
bilayer
- The helices in many cells are perpendicular to the bilayer, but in some the helices transverse the
membrane at an oblique angle
Lipid-Anchored Membrane Proteins
- bound covalently to one or more lipid molecules
- The hydrophobic segment of the attached lipid is embedded in
one leaflet of the membrane and anchors the protein to the
membrane
- The polypeptide chain itself doesn’t enter the bilayer
- Proteins anchored to membrane by lipophilic adduct
- Acylation of Gly residue of protein
- Prenylation of Cys residue of protein
- Glycosylphosphatidylinositol (GPI) anchor (exoplasmic)
- Acylation attaches through N-terminal Gly residue
- Prenylation attaches Cys residue at or near C terminus
- In these lipid proteins the lipid hydrocarbon chains are embedded in the bilayer, but the protein
itself does not enter the bilayer
- The anchors used to insert proteins at the cytosolic face are not used for the exoplasmic face and
vice versa
- One group of cytosolic proteins are anchored to the cytosolic face of a membrane by a fatty acyl
group covalently attached to an N-terminal gylcine residue
- Retention of such proteins at the membrane by the N terminal acyl anchor, called acylation may
play an important role in a membrane associated function
- A second group of cytosolic proteins are anchored to membranes by a hydrocarbon chain attached
to a cysteine residue at or near the C terminus
- Some of these chains are prenyl anchors built from 5-carbon isoprene units, which are also used in
the synthesis of cholesterol
- The additional hydrocarbon anchor is thought to reinforce the attachment of the protein membrane
- Some cell surface proteins and specialized proteins with distinctive covalently attached
polysaccharides called proteoglycans are bound to the exoplasmic face of the PM by a third type
of anchor group, GPI
- The exact structures of GPI anchors vary greatly in different cell types, but they always contain
phosphatidylinositol (PI), whose two fatty acid chains extend into the lipid bilayer just like those
of typical membrane phospholipids; PE, which covalently links the anchor to the C terminus of a
protein; and several sugar residues
- The enzyme phospholipase C cleaves the phosphate-glycerol bond in PLs and in GPI anchors
Peripheral Membrane Proteins
- don’t directly contact the hydrophobic core of the bilayer
- non-covalent bonds; ionic or hydrogen bonds & van der waal forces
- Bound either indirectly or via interactions with integral proteins, lipid anchored proteins or
directly by interactions with lipid head groups
- Can be bound either cytosolic or exoplasmic
- Act as adapter proteins to interact with cytoskeletal filaments associated with the cytosolic face
(ankyrin and dystrophyn); interactions provide support for various cell membranes, help
determine cell shape and mechanical properties and can play a role in communication between the
interior and exterior of the cell
- Proteins on the outer surface of the PM and the exoplasmic domains of integral membrane
proteins are often attached to components of the extracellular matrix or to the cell wall
surrounding bacterial and plant cells, providing a crucial interface between the cell and its
environment
Insertion of Proteins into the Membranes
- Topology: orientation of a transmembrane protein
- Topogenic Sequences: 1. N terminal (cleaved) sequences, 2. Stop-transfer/membrane anchor
sequence (STA), 3. Signal Anchor – internal (uncleaved) sequence (SA)
Synthesis of Type I Proteins
- all type one transmembrane proteins possess an N terminal signal sequence that targets them to the
ER as well as an internal hydrophobic sequence that becomes the membrane spanning alpha helix
- the N terminal sequence on a nascent type one protein initiates co-translational translocation of the
protein through the combined action of the SRP and SRP receptor
-Once the N terminal enters the ER it is cleaved
- Once it has been cleaved the protein continues to translate into the ER lumen
- Unlike secretory proteins, there is a section of 22 hydrophobic amino acids that will become the
transmembrane portion of the protein, when this reaches the translocon, it stops transfer and the
rest of the protein is made in the cytoplasm of the cell.
- Stop Transfer Anchor Sequence: prevents the continued translation of a protein into the ER.
Becomes anchored in the membrane.
- 1. N-terminal is cleaved
- 2&3. The chain is elongated until the hydrophobic stop transfer anchor sequence is synthesized
and enters the translocon, which then prevents the further translation of the protein into the ER
- 4.The STA sequence moves laterally between the translocon subunits and becomes anchored in
the bilayer
- 5. As synthesis continues, the elongating chain may loop out into the cytosol through the small
space between the ribosome and the translocon
- 6. When translation is complete the ribosomal subunits are released into the cytoplasm leaving the
protein to diffuse in the membrane
Synthesis of Type II and III Proteins
- no cleavable N terminus, but both contain a
single internal hydrophobic signal anchor
sequence that functions as both an ER signal
sequence and a membrane signal sequence
- have opposite orientations into the membrane;
this corresponds with where there SA
sequences are in the translocon
-Type II protein SA sequence directs insertion
of the nascent chain into the ER membrane so
that the N terminus of the chain faces the
cytosol, using the same SRP dependent
mechanism described for signal sequences.
However SA is NOT cleaved and moves

Document Summary

Three types of membrane proteins: integral, lipid-linked, peripheral. The predicted length of such an alpha helix is just sufficient to span the hydrocarbon core of the bilayer. The helices in many cells are perpendicular to the bilayer, but in some the helices transverse the membrane at an oblique angle. Lipid-anchored membrane proteins bound covalently to one or more lipid molecules. The hydrophobic segment of the attached lipid is embedded in one leaflet of the membrane and anchors the protein to the membrane. The polypeptide chain itself doesn"t enter the bilayer. Prenylation attaches cys residue at or near c terminus. In these lipid proteins the lipid hydrocarbon chains are embedded in the bilayer, but the protein itself does not enter the bilayer. The anchors used to insert proteins at the cytosolic face are not used for the exoplasmic face and vice versa.