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Biology 2382B Lecture Notes - Action Potential, Lipophilicity, Ion Channel

Course Code
BIOL 2382B
Jessica Kelly

of 17
Lecture 14 Membrane Proteins
Since pure phospholipid membranes are only semipermeable to a few small molecules
and gases…and also function only to form a closed compartment…many biological
functions of membranes are carried out by membrane embedded/associated proteins…
1. Integral Membrane Proteins
Transmembrane Domain:
Hydrophobic secondary or tertiary structures that span the lipid bilayer
Commonly α helices (approx. 20-25 amino acids long)
Arg and Lys (charged AAs) near cytosolic side (+)ive charge cannot cross
membrane of ER during synthesis = hydrophilic (interact w/ polar head groups)
β barrels (another transmembrane structure composed of layered beta sheets)
Mostly glycosylated in exoplasmic domain… (Provides specificity for extracellular
Three Types of Membrane Proteins:
1. Integral
2. Lipid-linked
3. Peripheral
All are asymmetric... (i.e. not found in equal
abundance on both sides but rather
localized to one leaflet or the other…)
Water is on outside. Inside is hydrophobic,
outside is hydrophilic
Asymmetric specifically
Three Distinct Domains”:
Cytoplasmic (hydrophilic)
Exoplasmic (hydrophilic)
2. Lipid Linked Proteins
These proteins are anchored to the membrane by lipophilic adduct…
a) Acylation of Gly residue of protein @ N-terminus (palimate or other fatty acid)
b) Prenylation of Cys residue of protein @ C-terminus (5C isoprene units)
c) Glycosylphosphatidylinositol (GPI) anchor PI lipid is covalently linked to
sugar residues which are linked to phosphoethanolamine…this acts an anchor for
a protein and is exclusively exoplasmic
3. Peripheral (non-covalent weak interaction) Proteins
“Attached” through non-covalent interactions (relatively weak interactions)
Ionic Interactions
Hydrogen Bonds
Protein-protein Interactions
Van der Waals forces
Can act as adapter proteins and allow cytoskeletal filaments to associate with bilayer…
(i.e. dystrophin & ankyrin that link integral membrane proteins to cytoskeleton)
see diagram at beginning**
Lipid linked proteins do not
enter bilayer
Lipid linked proteins (&
most membrane proteins)
have lateral mobility within
the membrane
Insertion of Proteins into Membranes
Topogenic Sequences internal signal sequences of amino acids in a nascent peptide
that direct its insertion into the target membrane/organelle by directing the translating
ribosome to a translocon on the target membrane…they are recognized by their topology
(i.e. charge & shape) & vary from protein to protein
Three Types of Topogenic Sequences:
1. N-terminal signal sequence signal sequence at the N-terminus that is cleaved
upon entry into the ER lumen
2. Stop-Transfer Anchor Sequence (STA) uncleaved & internal signal sequence
composed of hydrophobic amino acid that becomes the membrane spanning -
3. Signal-Anchor Sequence (SA) uncleaved & internal signal sequence
composed of hydrophobic amino acid that becomes the membrane spanning -
helix (whether C or N terminus is in cytosol determined by location of (+)ive
charged AA’s relative to STA)
STA sequences stop the transfer (i.e. stop translation) into the lumen of the peptide as
well as form an
-helix transmembrane domain…
**ALL proteins begin being translated in the cytosol & require a signal in order to
transfer the protein to another organelle/organelle membrane…**