Essential Cell Biology 5th edition

502 CHAPTER 15 Intracellular Compartments and Protein Transport
TABLE 15–3 SOME TYPICAL SIGNAL SEQUENCES
Function of Signal
Import into ER
Example of Signal Sequence
+ H 3 N-Met-Met-Ser-Phe-Val-Ser-Leu-Leu-Leu-Val-Gly-
Ile-Leu-Phe-Trp-Ala-Thr-Glu-Ala-Glu-Gln-Leu-Thr-
Lys-Cys-Glu-Val-Phe-Gln-
Retention in lumen of ER -Lys-Asp-Glu-Leu-COO –
Import into mitochondria
Import into nucleus
Export from nucleus
Import into peroxisomes
+ H 3 N-Met-Leu-Ser-Leu-Arg-Gln-Ser-Ile-Arg-Phe-
Phe-Lys-Pro-Ala-Thr-Arg-Thr-Leu-Cys-Ser-Ser-Arg-
Tyr-Leu-Leu-
-Pro-Pro-Lys-Lys-Lys-Arg-Lys-Val-
-Met-Glu-Glu-Leu-Ser-Gln-Ala-Leu-Ala-Ser-Ser-Phe-
-Ser-Lys-Leu-
Positively charged amino acids are shown in red and negatively charged amino
acids in blue. Important hydrophobic amino acids are shown in green.
+ H 3 N indicates the N-terminus of a protein; COO – indicates the C-terminus.
described. These proteins are ferried by transport vesicles, which pinch
off from the membrane of one compartment and then fuse with the
membrane of a second compartment (mechanism 3 in Figure 15–5). In
this process, transport vesicles deliver soluble cargo proteins, as well
as the proteins and lipids that are part of the vesicle membrane.
Signal Sequences Direct Proteins to the Correct
Compartment
The typical sorting signal on a protein is a continuous stretch of amino
acid sequence, typically 15–60 amino acids long. This signal sequence is
often (but not always) removed from the finished protein once it has been
sorted. Some of the signal sequences used to specify different destinations
in the cell are shown in Table 15–3.
Signal sequences are both necessary and sufficient to direct a protein to
a particular destination. This has been shown by experiments in which
the sequence is either deleted or transferred from one protein to another
by genetic engineering techniques (discussed in Chapter 10). Deleting a
signal sequence from an ER protein, for example, converts it into a cytosolic
protein, while placing an ER signal sequence at the beginning of a
cytosolic protein redirects the protein to the ER (Figure 15–6). The signal
sequences specifying the same destination can vary greatly even though
cytosolic protein
(no signal sequence)
ER protein with signal
sequence removed
Figure 15–6 Signal sequences direct
proteins to the correct destination.
(A) Proteins destined for the ER possess
an N-terminal signal sequence that directs
them to that organelle, whereas those
destined to remain in the cytosol lack any
such signal sequence. (B) Recombinant
DNA techniques can be used to change the
destination of the two proteins: if the signal
sequence is removed from an ER protein
and attached to a cytosolic protein, both
proteins are reassigned to the expected,
inappropriate location.
(A)
ER
cytosol
ER protein ER signal sequence cytosolic protein with
added ER signal sequence
NORMAL SIGNAL SEQUENCE
ER signal
sequence removed
from ER protein
and attached
to cytosolic
protein
(B)
ER
RELOCATED SIGNAL SEQUENCE