Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

FROM RNA TO PROTEIN
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which directs the ubiquitylation of the protein and sends it to the proteasome
for degradation. Thus, the majority of human proteins carry their own signals for
destruction. It has been proposed that when a protein is properly folded (and,
before that, when it is in contact with a chaperone), this acetylated N-terminus is
buried and therefore inaccessible to the E3 enzyme. According to this idea, as a
protein ages and becomes damaged (or if it fails to fold correctly from the start),
this destruction signal becomes exposed, and the protein is destroyed.
There Are Many Steps From DNA to Protein
We have seen so far in this chapter that many different types of chemical reactions
are required to produce a properly folded protein from the information contained
in a gene (Figure 6–87). The final level of a properly folded protein in a cell therefore
depends upon the efficiency with which each of the many steps is performed.
We also now know that the cell devotes enormous resources to selectively degrading
proteins, particularly those that fail to fold properly or accumulate damage
as they age. It is the balance between the rates of synthesis and degradation that
determines the final amount of every protein in the cell.
In the following chapter, we shall see that cells have the ability to change the
levels of their proteins according to their needs. In principle, any or all of the steps
in Figure 6–87 could be regulated for each individual protein. As we shall see in
Chapter 7, there are examples of regulation at each step from gene to protein.
5′
3′
introns
exons
DNA
INITIATION OF TRANSCRIPTION
CAPPING,
ELONGATION
SPLICING
cap
CLEAVAGE,
POLYADENYLATION,
AND TERMINATION
AAAA
mRNA
EXPORT
poly-A tail
NUCLEUS
AAAA
mRNA
CYTOSOL
mRNA DEGRADATION
INITIATION OF PROTEIN SYNTHESIS (TRANSLATION)
AAAA
COMPLETION OF PROTEIN SYNTHESIS
AND PROTEIN FOLDING
NH 2
COOH
NH 2
COOH
PROTEIN DEGRADATION
Figure 6–87 The production of a protein
by a eukaryotic cell. The final level of each
protein in a eukaryotic cell depends upon
the efficiency of each step depicted.