Cell Biology Lecture No. 19: Membrane Proteins
Wednesday March 20 , 2013
LECTURE 18 CONT’D
-The plasma membrane, a single bilayer membrane, encloses the cell. This bilayer is made up of leaflets
that have two faces: the cytosolic face and the cytosolic face (side of the membrane in contact with the
cytosol) and the exoplasmic face (side of the membrane in contact with the extracellular medium). The
cytosolic face for the plasma membrane is the internal face or portion of the bilayer, while vesicles and
some organelles have the external face as their cytosolic face. The PM, vesicles and most organelles
have a single membrane bilayer, while organelles like the nucleus, mitochondrion, and chloroplast are
enclosed by two membranes separated by a small intermembrane space. The exoplasmic faces of the
inner and outer membranes around these organelles border the intermembrane space between them.
-The faces of cellular membranes are conserved during membrane budding and fusion. During
endocytosis a segment of the plasma membrane buds inward toward the cytosol and eventually pinches
off a separate vesicle. During this process the cytosolic face of the plasma membrane remains facinq the
cytosol and the exoplasmlc face of the new vesicle membrane faces the vesicle lumen. During exocytosis
an intracellular vesicle fuses with the plasma membrane, and the lumen of the vesicle (exoplasmic face)
connects with the extracellular medium. Proteins that span the membrane retain their asymmetric
orientation during vesicle budding and fusion; in particular the same segment always faces the cytosol.
Semi-Permeability & Protein Asymmetry:
-The relative permeability of a pure phospholipid bilayer differs for various molecules and ions. A bilayer
is permeable to many gases and to small, uncharged or hydrophobic molecules. It is slightly permeable
to water, and essentially impermeable to ions and to large, charged or hydrophilic molecules.
-There is much asymmetry present within a biomembrane: Phospholipid composition differs between
leaflets, carbohydrates are found exclusively on the exoplasmic face, proteins are either embedded in
the bilayer in a fixed orientation or are associated with only one side, etc.
-With the exception of needing phospholipids to form semi- permeable closed compartments,
membrane proteins carry out the unique biological functions of specific membranes. There are three
types of membrane-associated proteins, all of them exhibiting asymmetry: integral membrane proteins,
lipid-linked proteins and peripheral membrane proteins. Integral Membrane Proteins:
-Integral membrane proteins are asymmetric, specifically oriented protein molecules that are
permanently embedded in the plasma membrane. Integral membrane proteins have three distinct
domains: a cytoplasmic (hydrophilic) domain, transmembrane (hydrophobic) domain and an exoplasmic
(hydrophilic) domain. Two most common types of transmembrane domains are α-helices and β-barrels.
α-helices consist of a row of about 20-25 hydrophobic amino acids that span the lipid bilayer. On the
cytosolic domain of integral membrane proteins are positively-charged amino acids near its
transmembrane domain that basically help anchor the protein by interacting with polar head groups.
Most integral membrane proteins have their exoplasmic domains glycosylated (addition of sugars) in the
-Lipid-linked membrane proteins are proteins anchored to the plasma membrane by a lipophilic adduct.
These proteins do not enter the bilayer (they are either present in the cytosol or in the extracellular
medium), but are capable of lateral mobility in the membrane. GPI (Glycosylphosphatidylinositol)
anchors are always exoplasmic (linking to proteins exterior to the cell) and consist of two components:
phosphatidylinositol (phosphoglyceride present in the PM) and sugar residues (link phosphatidylinositol
to membrane protein). Recall that N-CAM proteins of the Ig superfamily utilize GPI anchors. For lipid-
linked proteins present in the cytosol, membrane association is achieved through two ways: acylation
and prenylation. Acylation attaches a glycine residue at the N-termin