Essential Cell Biology 5th edition

258 CHAPTER 7 From DNA to Protein: How Cells Read the Genome
newly synthesized
polypeptide chain
FOLDING AND
COFACTOR BINDING,
DEPENDENT ON
NONCOVALENT
INTERACTIONS
COVALENT MODIFICATION
BY, FOR EXAMPLE,
PHOSPHORYLATION
NONCOVALENT BINDING
TO OTHER PROTEIN
SUBUNIT
P
where they are translated to produce a protein. But the process does not
end there. Proteins must then fold into the correct, three-dimensional
shape (as we discuss in Chapter 4). Some proteins do so spontaneously,
as they emerge from the ribosome. Most, however, require the assistance
of chaperone proteins, which steer them along productive folding pathways
and prevent them from aggregating inside the cell (see Figures 4–8
and 4–9).
In addition to folding properly, many proteins—once they leave the ribosome—require
further adjustments before they are useful to the cell. As
we discussed in Chapter 4, some proteins are covalently modified—for
example, by phosphorylation or glycosylation. Others bind to smallmolecule
cofactors or associate with additional protein subunits. Such
post-translational modifications are often needed for a newly synthesized
protein to become fully functional (Figure 7–45). The final concentration
of a protein, therefore, depends on the rate at which each of these steps—
from DNA to mature, functional protein—is carried out (Figure 7–46).
In principle, any one of these steps can be controlled by cells as they
adjust the concentrations of their proteins to suit their needs. However,
P
5′
3′
promoter
introns
exons
DNA
mature functional protein
Figure 7–45 Many proteins require posttranslational
modifications to become
fully functional. To be useful to the cell, a
completed polypeptide must fold correctly
into its three-dimensional conformation
and then bind any required cofactors (red)
and protein ECB5 partners—all e7.43/7.46 via noncovalent
bonding. Many proteins also require one
or more covalent modifications to become
active—or to be recruited to specific
membranes or organelles (not shown).
Although phosphorylation and glycosylation
are the most common, more than 100 types
of covalent modifications of proteins are
known.
RNA
transcript
5′ cap
INITIATION OF TRANSCRIPTION
intron sequence
5′ RNA CAPPING, ELONGATION,
AND SPLICING OF FIRST INTRON
intron sequence
ADDITIONAL INTRONS SPLICED,
3′ POLYADENYLATION, AND
TERMINATION OF TRANSCRIPTION
EXPORT
AAAA mRNA
poly-A tail
NUCLEUS
AAAA
mRNA
CYTOSOL
mRNA DEGRADATION
INITIATION OF TRANSLATION
AAAA
COMPLETION OF TRANSLATION
AND PROTEIN FOLDING
Figure 7–46 Protein production in a
eukaryotic cell requires many steps. The
final concentration of each protein depends
on the rate of each step depicted. Even after
an mRNA and its corresponding protein have
been produced, their concentrations can be
regulated by degradation.
POST-TRANSLATIONAL
MODIFICATION
pool of functional protein
PROTEIN DEGRADATION
P