Biology Reference
In-Depth Information
premature termination codon; and, perhaps, mRNA localization. For example,
during development of
Drosophila
, cellularization of the blastoderm can result
in the destabilization of the
bicoid
mRNA.
2.13 Chaperones and the Proteosome
Maintaining the function of proteins within cells depends on more than transcrip-
tion and translation. The initial folding of proteins and assembly of multiprotein
complexes sometimes requires the assistance of molecular
chaperones
—proteins
that catalyze protein folding. Chaperones prevent proteins from aggregating
into insoluble, nonfunctional blobs and help them reach and maintain a stable
functional state. Understanding of protein folding has increased dramatically
in the past few years (
Radford 2000
). There seems not to be a single, specific fold-
ing pathway; rather, there are potentially a plethora of routes to the folded pro-
tein and which pathways are used will depend on the amino-acid sequence, the
topology of the protein, and the environmental conditions of the cell. Different
routes might be used or different intermediates and transition states might
be observed as a consequence of relatively small alterations of in the cellular
environment.
After initial folding and assembly, proteins may be damaged. Such proteins
can be rescued (refolded) by chaperones or are destroyed by proteases. The effi-
ciency and cost of protein quality depend on a balance among folding, refold-
ing, and degradation (
Wickner et al. 1999
).
Controlled degradation of proteins inside the cell is essential. For example,
protein degradation is important in cell cycle control, heat-shock response, pro-
grammed cell death, muscle atrophy, immune response, metamorphosis, devel-
opment, and differentiation (
Mykles 1999
). Proteins targeted for destruction
are marked by the attachment of a small protein called
ubiquitin
and, after
unfolding, the protein is degraded within a large protein complex known as the
proteosome
(
Stuart and Jones 1997
). Protein degradation is potentially hazard-
ous to the cell and must be restricted to specific sites and times to prevent the
improper destruction of useful proteins. The proteosome provides a compart-
ment to confine the proteolytic action to proteins that carry the degradation
signal.
2.14 RNA Silencing or Interference and miRNAs
RNA silencing
(also called
RNA interference
[RNAi]) is a mechanism for defend-
ing against the invasion of mobile DNA elements (transposable elements)
that can cause mutations when they insert themselves into or close to a gene
