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Chapter 5 Protein Synthesis, Processing, and Trafficking 57

the plasma membrane lipids and proteins that are lost by ongoing set of cargo receptors is linked to vesiculation resulting in concentra-
secretory activity. There are three types of endocytosis: (1) phagocy- tion of the cargo. The coated pit pinches off from the plasma
tosis (cell eating), (2) pinocytosis (cell drinking) and (3) receptor- membrane by the action of a GTP-binding protein, dynamin, which
mediated endocytosis. Defects in endocytosis can underlie human forms a ring around the neck of each bud and contributes to the
diseases. For example, patients with familial hypercholesterolemia vesicle formation. After release and shedding of the clathrin coat, the
(FH) have elevated serum cholesterol because of mutations in the vesicle fuses with the EE compartment.
low-density lipoprotein (LDL) receptor that prevents the endocytic
uptake of LDL and its catabolism in lysosomes.
SPECIFICITY OF VESICULAR TARGETING

Phagocytosis As described earlier, COPI-, COPII-vesicles transport material early
in the secretory pathway whereas clathrin-coated vesicles transport
During phagocytosis, cells are able to ingest large particles (greater than material between the plasma membrane and Golgi. Coating proteins
0.5 µm in diameter) which serves not only to engulf and destroy assemble at specific areas of the membrane in a process controlled by
invading bacteria and fungi but also to clear cellular debris at wound the coat-recruitment GTPases: Arf1 is responsible for the assembly
sites and to dispose of aged erythrocytes. Primarily, specialized cells, of COPI coats and clathrin coats at Golgi membranes whereas
including macrophages, neutrophils and dendritic cells, execute Sar1p is responsible for COPII coat assembly at the ER membrane.
phagocytosis. Phagocytosis is triggered when specific receptors contact In yeast, the process of vesiculation in the transport from the ER
structural triggers on the particle, including bound antibodies, to Golgi has been dissected at a molecular level. On the cytosolic
complement components as well as certain oligosaccharides. Then face of the ER membrane, Sar1p is activated by the ER localized
actin polymerization is stimulated, driving the extension of pseudo- GEF Sec12p. Sar1-GTP assembles with the Sec23-Sec24 complex
pods, which surround the particle and engulf it in a vacuole called whose Sec24 subunit binds directly or through a membrane receptor
phagosome. The engulfed material is destroyed when the phagosome to specific signals displayed by the cargo. This prebudding cargo
fuses with a lysosome, exposing the content to hydrolytic enzymes. complex recruits the outer layer Sec13-Sec31 complex leading to
In addition, phagocytosis is a means of “presenting” the pathogen’s coat polymerization, membrane deformation and COPII-vesicle
components to lymphocytes, thus eliciting an immune response. formation.
Clathrin-coat assembly at the plasma membrane is also thought
to involve a GTPase but its identity is unknown. These regulatory
Pinocytosis proteins also ensure that membrane traffic to and from an organelle
are balanced.
Pinocytosis refers to the constitutive ingestion of fluid in small pino- After budding, vesicles are transported to their final destination
cytotic (endocytic) vesicles (0.2 µm in diameter) and occurs in all by diffusion or motor-mediated transport along the cytoskeletal
cells. Following invagination and budding, the vesicle becomes part network (microtubules or actin). The molecular motors kinesin,
of the endosome system that is described later. The plasma membrane dynein and myosin have been implicated in this process. The vesicles
portion that is ingested returns later through exocytosis. In some cells, undergo an uncoating process before fusion with the correct target
pinocytosis can result in turnover of the entire plasma membrane in membrane. Both transport vesicles and target membranes display
less than 1 hour. surface markers that selectively recognize each other.
Three classes of proteins guide the selectivity of transport vesicle
docking and fusion: (i) complementary sets of vesicles SNAREs,
Receptor-Mediated Endocytosis v-SNAREs, (soluble NSF association protein receptor) and target
membrane SNAREs (t-SNAREs) that are crucial for the fusion, (ii)
Receptor-mediated endocytosis is a means to import macromolecules a class of GTPases, called Rabs, and (iii) protein complexes called
from the extracellular fluid. More than 20 different receptors are tethers that, together with Rabs, facilitate the initial docking of the
internalized through this pathway. Some receptors are internalized vesicles to the target membrane.
continuously whereas others remain on the surface until a ligand is Rab GTPases function as the master regulators of membrane
bound. In either case, the receptors slide laterally into coated pits that traffic but are themselves regulated by factors that control their activa-
are invaginated regions of the plasma membrane surrounded by tion by GEFs or their inactivation by GAP, proteins that stimulate
clathrin and pinch off to form clathrin-coated vesicles. The immedi- the intrinsic GTPase activity.
ate destination of these vesicles is the endosome.
The endosome is part of a complex network of interrelated
membranous vesicles and tubules termed the endolysosomal system. CONCLUSIONS
The endolysosomal system comprises four types of membrane-bound
structures: early endosomes (EEs), late endosomes (LEs), recycling The mechanisms regulating protein synthesis, processing, degrada-
vesicles, and lysosomes. It is still a matter of debate whether these tion and transport are under intense investigation. Protein motifs and
structures represent independent stable compartments or one struc- their cognate receptors have been identified for many intracellular
ture matures into the next. The interior of the endosomes is acidic sorting and processing reactions. Studies are now directed to elucidate
(pH about 6). Endocytosed material is ultimately delivered to the these processes at a molecular level by resolution of the three dimen-
lysosome, presumably by fusion with LE. Lysosomes also digest sional structures of the proteins involved in protein processing and
obsolete parts of the cell in a process called autophagy. trafficking. The future challenge will be to find ways of exploiting
During the formation of clathrin-coated vesicles, clathrin mol- this knowledge to intervene in the numerous disease states that result
ecules do not recognize cargo receptors directly but rather through from errors in these processes.
the adaptor proteins, that form an inner coat. The AP-2 components
bind both clathrin and sorting signals present in the cytoplasmic tails
of cargo receptors close to the plasma membrane. These internaliza- SUGGESTED READINGS
tion motifs are: YXXϕ (where ϕ is a hydrophobic amino acid), as a
most common motif, and the NPXY signal that was first identified Amm I, Sommer T, Wolf DH: Protein quality control and elimination of
in the LDL receptor. For receptors that are internalized in response protein waste: the role of the ubiquitin-proteasome system. Biochim
to ligand binding, the internalization signal may also be generated by Biophys Acta 1:182–2014, 1843.
a conformational change induced by the binding of the ligand. Bagola K, Mehnert M, Jarosch E, et al: Protein dislocation from the ER.
Through the specificity of the AP-2 complex, the capture of a unique Biochim Biophys Acta 3:925–2011, 1808.