Biomedical Engineering Reference
In-Depth Information
Fig. 1
Schematic representation of the processes of intracellular protein traffi cking.
The distribution of proteins in eukaryotic cells relies on the translocation of
proteins across membranes (A-E) and the dynamics of membrane structures,
particularly the coordinated exchange of vesicles (F-P). Whereas endosymbiont-
derived organelles (mitochondria and chloroplasts) are presumably rather well-
defi ned structures, all other cellular compartments form an intricately connected
endomembrane system that is in constant fl ux thereby vibrantly exchanging lipid
and protein constituents. Given the obvious complexity of these processes one
can only wonder how intracellular communication in eukaryotic cells avoids a
Babylonian confusion
chaperones which usher their cargo to appropriate translocases at
the target membrane. These translocases mediate the transfer of
the polypeptides across, or their insertion into the membrane of the
target compartment, where proteins are then folded into their
native three-dimensional structures with the help of chaperones.
Proteins destined for the endomembrane system are initially trans-
located into the ER, where they accumulate at dedicated budding
sites (ER exit sites; ERES) to embark on a journey along the secre-
tory pathway [
7
-
10
]. Coat proteins that are recruited by small
GTPases onto the surface of the ER mediate the budding of vesicles
and are released before the vesicles fuse with the Golgi. The Golgi
serves as a sorting station from which vesicles destined for various
subcellular locations bud off. An individual protein might pass
through several different sorting events before it reaches its fi nal
destination.
