membrane structure proteins

4 types of phospholipids

• phosphatidylethanolamine

• Phosphatidylserine

• Phosphatidylcholine

• Phosphatidylinositol - has inositol attached

sphingomyelin

• sphingosine backbone

• 2 hydrocarbon chains

• choline attached to phosphate

glycolipids

• galactocerebroside

• gangliosides (more complex chemically)

where is there a high abundance of glycolipids

• neuronal membranes 

cholesterol structure

• polar OH group

• ring steroid - rigid

• hydrocarbon tail 

• inc stability, red permability 

lipid to protein proportions

• 50 lipid molecules to one protein

• Proteins contribute more to mass 30-45%

composition of the leaflets

• outer leaflet - PC, SM, glycolipids, involved in cell contact

• Inner leaflet - PE, Pi, PS, involved in signalling

flippases

from the extracellular/outer to intracellular/inner leaflet P4-type-ATPase (P1-type = Na+/K+-ATPase ) moves PS/PE

floppases

from the intracellular/inner to extracellular/outer leaflet ABC ATPase from ABC (ATP-binding cassette) transporter superfamily

impact of loss of lipid symmetry

• ps on outer leaflet signals cell to be engulfed

• inhibition of flippases

• activation of calcium dependent phospholipid scramblases

when is increase of PS seen

• oxidative stress

• platelet plug formation in clotting

• immune system identification of cells infected by a virus

• sickle cell disease and associated erythrocyte pathology 

protein synthesis in the rough ER

• nascent protein - protein chain - is associated with a ribosome complex

• this complex binds to the SRP receptor and docks onto the translocon

• this creates channel interactions at the ER membrane 

• signal recognition particle temp inhibits protein synthesis then resumes once bound 

• parts can be cleaved and carbs can be added 

what causes the translocon to be associated with the polypeptide chain ribosome complex

signal sequence

n-acylation

attachment of a hydrocarbon chain to the N-terminal of the polypeptide sequence.

n-myristoylation definition

• anchors certain proteins to the cytosolic leaflet of the membrane with a 14C fatty acid myristoyl chain

where does n-myristoylation occur

• n terminal methionine amino acid residue is adjacent to a glycine amino acid residue in the polypeptide chain

what is N-Myristoylation catalysed by 

• N-myristoyltransferase (NMT)

Examples of proteins with the N-myristoylation modification:

• alpha -Subunits of G proteins.

• Catalytic subunit of PKA.

• Protein Phosphatase Calcineurin (PP2B).

s- acylation

attachment of hydrocarbon chains to S in the side chain of a Cysteine amino acid residue at the protein’s surface

S-Palmitoylation

anchors certain cytosolic proteins to the cytosolic leaflet of the membrane with a 16C fatty acid palmitoyl group attached to a cysteine at any point along the polypeptide sequence

examples s-palmitoylation

• GAD - glutamic acid decarboxylase (GABA synthesis from L-glutamate).

• t-SNARE - SNAP25 (vesicle fusion proteins).

S-isoprenylation

• anchors proteins to the cytosolic side of membranes

• hydrocarbon chain containing isoprenyl group repeats are attached to a cysteine at C terminal

examples of s-isoprenylation

• GTPases - Ras (signal transduction amplification)

• Rab (vesicle transport/cycling)

characteristics of intracellular anchoring 

• post translational modification

• bind reversible to the cytosolic leaflet

• not permanent 

• modifiable 

extracellular anchoring

GPI is added to the protein C terminal by peptide like linkage

• anchors protein in outer extracellular leaflet

where does extracellular anchoring occur

• within the lumen of ER

protein examples for extracellular anchoring

• AchE

• neural cell adhesion molecule

• ephrin-A ligands

how can proteins move in membrane

• spin about z acis

• distribute laterally - 2d instead of 3d

• cannot flip rotate or translocate

formation of functional barriers and fences

1. Aggregation of cells/proteins within the cell membrane

2. Tethering to macromolecules outside the cell

3. Tethering to macromolecules inside the cell

4. Interaction with proteins on adjacent cells

formation of lipid rafts

• segregation/stabilisation of lipids and proteins

• more organised microdomains

• protein rich

• tightly packed with inc cholesterol, sphingomyelin, saturated fatty acids

• GPI anchored proteins

• associated with CAVEOLIN proteins

what create a thicker membrane

• saturated fatty acids staighten the membrane 

formation of CAVEOLAE

• membrane is associated with CAVEOLIN protein

• creates a pit in the membrane

significance of lipid rafts and caveolae

• facilitate protein trafficking

• regulate endocytosis and exocytosis

• external signalling - growth factors, immune cell interactions

• internal signalling - GPCR, ion channels

• metabolic localisation