|Cells contain numerous metabolic compartments
and membranes that have unique sets of proteins. Nearly
all of these proteins are synthesized in the cytosol.
How do they reach their correct destinations? Proteins
have sorting domains that act as address labels; there
is a unique label for each destination. These sorting
domains are usually short segments of polypeptide that
may or may not be removed after the proteins reach their
Many sorting domains interact
with proteinaceous pores in membranes. Such pores consist
of numerous different polypeptides and are unique to each
organelle. Passage of a polypeptide through a pore requires
the participation of chaperones and energy in the form
of ATP, GTP, or both. Chaperones help keep proteins unfolded
in the cytosol before passage through the pore and are
required to fold the protein correctly once it has crossed
the membrane. All living cells maintain an active secretory
pathway having several functions besides protein secretion.
Proteins move between the various compartments of this
pathway as cargo in vesicles. All proteins that have a
signal peptideare translated on rough ER and enter the
secretory pathway via translocation into the ER lumen.
Correct folding is necessary not only for proper protein
function but also for transport. Proteins that are not
correctly folded are degraded in the ER. Vesicle formation
and sorting are complex processes that involve integral
membrane proteins (e.g., receptors) and cytosolic proteins
(e.g., vesicle coat proteins). Coat formation is necessary
for vesicle budding. Vesicles always uncoat before fusing
with their target compartments Cells make extensive use
of highly regulated GTP-binding proteins/GTPases to regulate
and provide energy for secretory pathway activity. The
secretory pathway targets proteins to the ER, Golgi, TGN,
prevacuolar compartment, vacuole, and plasma membrane.
Cells also have a vesicle-mediated mechanism for taking
up proteins from their surroundings and delivering them
to various compartments.