| Comments |
"ER sheets are relatively flat areas
where the membrane extends for
many microns with little membrane
curvature. Although the nuclear
envelope is spherical, the nucleus is
so large that its surface can also be
considered as a flat ER sheet. In contrast, ER tubules are long cylindrical
units with high membrane curvature
in cross-section (reviewed in Voeltz
et al., 2002). Both sheet and tubular
domains are present in all eukaryotes, from plants to humans (reviewed
in Staehelin, 1997), but they have various organizations in different cells
and species. In yeast, the peripheral
ER (that is, any ER other than the
nuclear envelope) is located close
to the cell cortex and has only a few
tubules connecting it to the nuclear
envelope- in higher eukaryotes, the
peripheral ER extends throughout
the entire volume of the cytoplasm. Regardless of the cell type or subcellular location, the thickness of a sheet and the diameter of a tubule is
typically 60–100 nm. This conserved
structural regularity of the domains suggests that both sheets and
tubules are being shaped actively." |