Summary MBOC Chapter 13 - Intracellular membrane traffic

Chapter 13: Intracellular Membrane Traffic
 exocytosis – transport vesicle fuses with the plasma membrane
 endocytosis – plasma membrane patch is internalised, forming a transport vesicle –
content derives from extracellular space
o endosomes = internal
compartments
 lumen = interior space of each membrane-
enclosed compartment
o proteins can travel in this space
without having to cross a membrane
 transport vesicles = forms of containers
o bud off from one membrane and
fuse with another, carrying membrane components and soluble luminal
molecules, which are referred to as cargo
o donor compartment and target compartment
o must be selective
 secretory pathway – ER  Golgi apparatus
 endocytic pathway – leads inward from the plasma membrane
 retrieval transport  transport in opposite direction; return to ER

The molecular mechanisms of membrane transport and the maintenance of
compartmental diversity
 coated vesicles  specialized, coated regions of membranes; have a distinctive
cage of proteins covering their cytosolic surface
o 1) concentrates specific membrane proteins in a specialised patch, which then
gives rise to the vesicle membrane
o 2) shapes vesicle
 Three well-characterised types of
coated vesicles, distinguished by
their major coat proteins:
o 1) clathrin-coated –
mediate transport form the
Golgi apparatus and from
the plasma membrane
o 2) COPI-coated – mediate
transport from the ER and
from the Golgi cisternae
 bud off from Golgi
apparatus
o 3) COPII-coated – mediate transport from the ER and from the Golgi cisternae
 bud off from ER
 Adaptor proteins select cargo into clathrin-coated vesicles

Transport from the ER through the Golgi apparatus
 Golgi apparatus = Golgi complex  major site of carbohydrate synthesis, as well
as a sorting and dispatching station for products of the ER
 Proteins leave the ER at ER exit sites -whose membrane lacks bound ribosomes
o Proteins leave the ER in COPII-coated transport vesicles
 Only proteins that are properly folded and assembled can leave the ER
o otherwise: chaperones / proteasomes

,  Vesicular tubular clusters mediate
transport from the ER to the Golgi
apparatus
o heterotypic fusion
o homotypic fusion
 retrieval pathway  carrying back ER
resident proteins that have escaped,
as well as proteins such as cargo
receptors and SNAREs that
participated in the ER budding and
vesicle fusion reactions
o uses ER retrieval signals
 The retrieval pathway to the ER uses sorting signals: KDEL
o on C-terminus!
o Lys-Asp-Glu-Leu
o KDEL receptor travel between ER and Golgi apparatus
 can interact both with COPII coat and COPI coat
 has high-affinity for KDEL sequence in vesicular tubular clusters and
the Golgi apparatus
 has low-affinity for KDEL sequence in ER




 Golgi apparatus
o cisternae  membrane-
enclosed compartments
o cis face = entry face  cis
Golgi network (CGN)
o trans face = exit face 
trans Golgi network (TGN)
o N-linked oligosaccharide
serve to help proteins fold
and to help transport
misfolded proteins to the
cytosol for degradation in
proteasomes
  controlling quality
of proteins exiting from the ER
 are processes in the Golgi apparatus
 Two broad classes of N-linked oligosaccharides:
o complex oligosaccharides