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A more contemporaneous classification divides the ER in three domains:
the nuclear envelope (NE), the sheet-like cisternae, and the polygonal array
of tubules ( Shibata et al., 2006 ). The NE controls the flow of information
between the cytoplasm and the nucleoplasm, and consists of a double mem-
brane enclosing a lumen. The NE surrounds the nucleus, with the inner and
outer membranes connected only at the nuclear pores, the former serving
as a scaffold for chromatin organization localized at the inner membrane
( Dreier and Rapoport, 2000 ).
In yeast, there are more ribosomes in the cisternae than in the tubules,
suggesting that either cisternae are better suited for ribosome binding and/
or ribosome binding stabilizes cisternal ER structure. In animal cells, the
protein translocation machinery is also enriched in this structure. For exam-
ple, the expression of p180, a yeast ribosome-binding protein anchored to
the ER membrane, increases secretory activity. Moreover, ectopic expres-
sion of p180 in nonsecretory mammalian cells induces biogenesis of RER
membrane, while upon p180 knockdown, THP-1 secretory cells decrease
biogenesis in the area occupied by RER ( Benyamini et al., 2009 ).
Reticular networks can be formed de novo in Xenopus egg extracts
studied in vitro, in a cytoskeleton-independent manner but dependent on
energy and protein elements which drive microsome fusion into tubule
formation ( Dreier and Rapoport, 2000 ). The integral membrane protein
Reticulon 4a (Rtn4a), a member of an ER membrane-localized protein
family, has been implicated in tubule formation. Consistent with this role,
Rtn4a is absent in sheets and NE, but enriched in ER tubules ( Voeltz et al.,
2006 ). The yeast protein,Yop1p, and its mammalian ortholog, DP1, com-
prise another family of integral membrane proteins that form oligomers on
the tubular ER and are fundamental for tubule formation ( Shibata et al.,
2008 ). Rtn4a expression generates more tubules, while Rtn4a depletion
along with DP1/Yop1 converts the peripheral ER into sheets ( Voeltz
et al., 2006 ).
In muscle cells, a variant of the ER, termed sarcoplasmic reticulum (SR)
is present, whose main function is to control calcium release for muscle
contraction. In striated muscle, the plasma membrane forms long invagina-
tions, called transverse tubules (T-tubules) that penetrate into the cytoplasm.
These structures insert between two terminal SR cisternae, forming a triad.
This highly organized T-tubule system is essential for rapid and precise
excitation-contraction coupling. Different proteins collaborate in T-tubule
biogenesis, and in the formation and maintenance of the triad in both car-
diac and skeletal muscles: caveolae are cell surface structures that initiate
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