Essential Cell Biology 5th edition

Protein Sorting
505
prospective nuclear protein
P
Ran-GDP
GDP
GTP IS HYDROLYZED,
Ran-GDP DISSOCIATES
FROM RECEPTOR
Ran-GDP
nuclear localization signal
PROTEIN BINDS
TO RECEPTOR
CYTOSOL
Ran-GAP
CYTOSOL
P
nuclear
pore
NUCLEUS
GTP
GTP
Ran-GEF
GDP
NUCLEUS
nuclear
import
receptor
(A)
Ran-GTP
PROTEIN DELIVERED
TO NUCLEUS
(B)
Ran-GTP
Ran-GTP BINDS
TO RECEPTOR
Figure 15–10 Energy supplied by GTP hydrolysis drives nuclear transport. (A) The small monomeric GTPase,
Ran, exists in two conformations—one carrying GTP, the other GDP (see Figure 4−48 or 16−12). Ran is converted
from one conformation to the other with the help of accessory proteins that are differently localized. The accessory
protein that triggers GTP hydrolysis, called Ran-GAP (GTPase-activating protein), is found exclusively in the cytosol,
where it converts Ran-GTP to Ran-GDP. The accessory protein that causes Ran-GDP to release its GDP and take up
GTP, called Ran-GEF (guanine nucleotide exchange factor), is found exclusively in the nucleus. The localization of
these accessory proteins guarantees that the concentration of Ran-GTP is higher in the nucleus, thus driving the
nuclear import cycle in the desired direction. (B) A nuclear import receptor picks up a prospective nuclear protein
in the cytosol and enters the nucleus. There it encounters Ran-GTP, which binds to the import receptor, causing it
to release the nuclear protein. Having discharged its cargo in the nucleus, the receptor—still carrying Ran-GTP—is
transported back through the pore to the cytosol, where ECB5 Ran e15.10/15.10 hydrolyzes its bound GTP. Ran-GDP falls off the import
receptor, which is then free to bind another protein destined for the nucleus. Ran-GDP is carried into the nucleus by
its own unique import receptor (not shown).
Proteins Unfold to Enter Mitochondria and Chloroplasts
Both mitochondria and chloroplasts are surrounded by inner and outer
membranes, and both organelles specialize in the synthesis of ATP.
Chloroplasts also contain a third membrane system, the thylakoid membrane
(discussed in Chapter 14). Although both organelles contain their
own genomes and make some of their own proteins, most mitochondrial
and chloroplast proteins are encoded by genes in the nucleus and are
imported from the cytosol. These proteins usually have a signal sequence
at their N-terminus that allows them to enter their specific organelle.
Proteins destined for either organelle are translocated simultaneously
across both the inner and outer membranes at specialized sites where
the two membranes are closely apposed. Each protein is unfolded as it
is transported, and its signal sequence is removed after translocation is
complete (Figure 15–11).
Chaperone proteins (discussed in Chapter 4) inside the organelles help to
pull the protein across the two membranes and to fold it once it is inside.
Subsequent transport to a particular site within the organelle, such as
the inner or outer membrane or the thylakoid membrane in chloroplasts,
usually requires further sorting signals in the protein, which are often
only exposed after the first signal sequence has been removed. The insertion
of transmembrane proteins into the inner membrane, for example, is
guided by signal sequences in the protein that start and stop the transfer
process across the membrane, as we describe later for the insertion of
transmembrane proteins in the ER membrane.
The growth and maintenance of mitochondria and chloroplasts require
not only the import of new proteins but also the incorporation of new