Essential Cell Biology 5th edition

Protein Sorting
501
proteins that are synthesized on ribosomes inside these organelles; most
mitochondrial and chloroplast proteins, however, are made in the cytosol
and subsequently imported. The fate of any protein molecule synthesized
in the cytosol depends on its amino acid sequence, which can contain
a sorting signal that directs the protein to the organelle in which it is
required. Proteins that lack such signals remain as permanent residents
of the cytosol; those that possess a sorting signal move from the cytosol
to the appropriate organelle. Different sorting signals direct proteins into
the nucleus, mitochondria, chloroplasts (in plants), peroxisomes, and
the ER.
When a membrane-enclosed organelle imports a water-soluble protein
to its interior—either from the cytosol or from another organelle—it faces
a problem: the protein must be transported across its membrane (or
membranes), which is normally impermeable to hydrophilic macromolecules.
How this task is accomplished depends on the organelle.
1. Proteins moving from the cytosol into the nucleus are transported
through the nuclear pores, which penetrate both the inner and outer
nuclear membranes. The pores function as selective gates that actively
transport specific macromolecules but also allow free diffusion of
smaller molecules (mechanism 1 in Figure 15–5).
2. Proteins moving from the cytosol into the ER, mitochondria, or
chloroplasts are transported across the organelle membrane by
protein translocators located in the membrane. Unlike the transport
through nuclear pores, the transported protein must usually unfold
for the translocator to guide it across the hydrophic interior of the
membrane (mechanism 2 in Figure 15–5). Bacteria have similar
protein translocators in their plasma membrane, which they use to
export proteins from the cytosol to the cell exterior.
3. Proteins moving onward from the ER—and from one compartment
of the endomembrane system to another—are transported by
a mechanism that is fundamentally different than the ones just
nucleus
1
TRANSPORT
THROUGH
NUCLEAR
PORES
chloroplast
mitochondrion
peroxisome
proteins
made in
cytosol
2
TRANSPORT
ACROSS
MEMBRANES
ER
endosome
Golgi
apparatus
3
TRANSPORT BY
VESICLES
Figure 15–5 Membrane-enclosed
organelles import proteins by one of
three mechanisms. All of these processes
require energy. The protein remains folded
during transport in mechanisms 1 and
3 but usually has to be unfolded during
mechanism 2.