Overview proteasomes (Proteasoma):
The images derive from a 20s proteasome of
an egg cell from Xenopus laevis.
Proteasomes (Terminologia histologica: Proteasoma)
are large protein complexes with a hollow cylindrical shape. Other regulatrory
proteins can be located at both ends of proteasome complex, whereas the
protein cleavage site is located in the central channel. There is still
discussion as to whether proteasomes are in reality cell
organelles or just large proteins; most favour the latter interpretation.
TEM images of proteasome have been produced from many different cell types.
3D image reconstructions reveal the molecular detail, with an
stereo image 1
stereo image 2
longitudinal + cross-section
average length of ~ 45 nm and diameter of ~13
nm. A long channel with a diameter of ~ 6nm (resembling
a pore when seen from above) is located in the centre of the cylindrial
molecule. Higher resolution X-ray structural detail is also avialable.
Proteasomes are present in both the cytoplasm
and nucleus of all eukaryotic cells, and in
the Archaeobacteria. Proteasomes play an important role in the following
processes: cellular differenciation, signal transduction,
of metabolic activity and cell division (mitosis)
by selective degradation of regulating and abnormal proteins.
Functionally proteasomes are proteinases, i.e. proteins that serve for
degradation of other proteins. Thereby specific enzymes link NH2-groups
to proteins that are destined to be destroyed in an energy-dependent manner
and link the marked proteins to several ubiquitin molecules. These ubiquitin
units draw the marked protein into the channel of a proteasome, where it
is split into peptides, while the linked ubiquitin molecules are recycled.
--> nucleus, cytoplasm,
--> Electron microscopic atlas Overview
--> Homepage of the workshop
I am very grateful to Prof.
Dr. rer. nat. R. Harris for the specimens. Images, page & copyright