Bacteriorhodopsin
Homotrimeric protein with 247 residues
Synthesized by halophillic archaea bacterium
Functions as proton pump, light driven
Retinal (vitamin A) = light absorption group
Transmembrane proteins
Alpha belies or beta sheets
Must fold to satisfy h-bonding potential of polypeptide backbone
Difficult to crystallize
Porins
Act as channels through which molecules can diffuse
Found in outer membrane of gram-negative bacteria mostly
Monomers or trimers
Minimum of 16-stranded antiparallel beta barrels that form central channel
3 types of lipid linked proteins
Prenylated, fatty acylated, GPI proteins
Prenylated Proteins
Its lipids are built from isoprene units
Most common prenylation site is the C-terminal tetra peptide C-X-X-Y
Y influences the prenylation; is Ala, Met or Ser OR Leu
Fatty acylated proteins
Myristic acid and palmitic acid = linked to membrane proteins
Many participate in transmembrane signaling
GPI-linked proteins
Occur in all eukaryotes, abundant in some parasitic Protozoa
Located on exterior of the protein
It’s core tetra saccharide varies depending on its linked protein
The fluid mosaic model
Accounts for lateral diffusion, integral proteins diffuse laterally in the lipid matrix, unless restricted by other cell components
Spectrin
Two similar polypeptide chains, an alpha beta dimer, each contain a triple stranded alpha helical coiled coil.
Form part of erythrocyte membrane exoskeleton
Spectrin
Accounts of 75% of erythrocyte membrane skeleton
Two similar polypeptide chains, an alpha beta dimer, each contain a triple stranded alpha helical coiled coil.
Flippases
Membrane protein that facilitates flip flop process by facilitated diffusion
Phospholipid transferase
Membrane protein that facilitates flip flop process by active transport
Lipid rafts
Consist of closely packed sphingolipids and cholesterol. Diffuse laterally within the membrane, certain proteins associate with the raft (including some that participate in transmembrane signaling)
Erythrocytes
Red Blood Cells
Translocon
Multifunctional trans-membrane pore
The channel-forming component of the translocon is a heterotrimeric protein named Sec61 in mammals and SecY in prokaryotes
Liposomes
Self-sealing solvent filled vesicles bound by a single bilayer.
Serve as models of biological membranes and also hold promise as vehicles for drug delivery.
Transverse diffusion
Transverse diffusion (a flip-flop) is defined as the transfer of a phospholipid molecule from one bilayer leaflet to the other by active transport (phospholipid translocases) or passive diffusion (flippases)
Clathrin
A protein that forms a polyhedral framework around vesicles that transport TM, GPI-linked, and secreted proteins from the Golgi to the plasma membrane.
Built from network of triskelions.
Lateral diffusion
The pairwise exchange of neighboring phospholipid molecules in the same bi-layer leaflet