Biology Reference
In-Depth Information
Two types of ER, rough and smooth, perform different cellular functions.
Rough ER is covered by ribosomes on its outer surface and functions in protein
processing (
Stutz and Rosbash 1998
). Smooth ER functions in lipid metabolism
(
Cooper 2000
). The details of ER transport are an active area of research.
Proteins move across the ER membrane through a hydrophilic channel that
is evolutionarily ancient (
Matlack et al. 1998
). The ER membrane channel trans-
locates proteins and also integrates membrane proteins into the lipid bilayer.
Thus, the ER membrane channel must identify the signal sequences of the pro-
teins, open in response to the signal, transport the protein from one side of the
membrane to the other, and then close. For membrane-spanning proteins, some
parts must be moved across the membrane, whereas others must be left in the
cytoplasm. The ER has several mechanisms to ensure that only properly folded
proteins enter the secretory pathway (
Reddy and Corley 1998
).
Proteins received from the ER are further processed and sorted for transport
to their eventual destinations in the
Golgi complex
(
Rothman 1994, Cooper
2000
). The Golgi complex is a cytoplasmic organelle specialized for processing
and sorting proteins and lipids before their transport to lysosomes, the plasma
membrane, or secretion. The Golgi is made of flattened membrane-enclosed
sacs (cisternae) and associated vesicles. Proteins from the ER enter one side of
the Golgi and exit from the opposite side. As the proteins pass through the
Golgi, they are modified; modifications that occur within the Golgi include
gly-
cosylation
. Glycosylation involves adding carbohydrates to proteins. Proteins,
lipids, and polysaccharides are transported from the Golgi complex to their final
destinations via different kinds of transport vesicles (
Featherstone 1998
). The
Golgi seems to be in constant flux, each cisterna emerging from the ER with its
load of proteins and then carrying the proteins across the Golgi, while simulta-
neously putting the finishing touches on the proteins.
2.12 mRNA Stability
Many of the genes involved in development are expressed for a short time and in
precisely defined domains of the body. Some genes affect the expression of genes
that are downstream in the developmental pathway. Thus, gene expression must
be precisely regulated if cell identity is to be specified. Modifying the rate of tran-
scription of DNA is not always sufficient to maintain fine-tuned developmental
processes. Sometimes existing mRNAs and proteins must be removed or deacti-
vated, tasks that involves regulating
mRNA stability
(
Surdej et al. 1994
).
Several mechanisms are involved in regulating mRNA stability, includ-
ing removal of the poly(A) tail; premature termination of translation due to a
