Biology Reference
In-Depth Information
apparatus, where protein sorting occurs. Proteins and lipids destined for the
endosomal/lysosomal system, the cell surface, or the extracellular space transit the
cisternae of the Golgi stacks in a directional manner and are glycosylated in a step-
wise process. In contrast, proteins that have ''escaped'' from the ER are recycled
back to the ER by retrograde transport. In the most trans cisternae, the Trans Golgi
Network (TGN), cargo molecules are packaged into transport carriers for delivery
to their final destinations.
In addition to the Golgi apparatus, elements of the ER-Golgi Intermediate
Compartment (ERGIC) have been detected in the pericentrosomal region (Marie
et al.
2009
). As the name implies, the ERGIC is a dynamic organelle at the interface
between ER exit sites and the cis Golgi, which functions as a major site of post-ER
protein sorting. Interestingly, the pericentrosomal domain of the ERGIC appears to
be independent of the Golgi, maintaining its close association with the centrosome
under conditions when Golgi membranes are dispersed (Marie et al.
2009
).
The Endocytic Recycling Compartment (ERC) is also found in the pericen-
trosomal region (Lin et al.
2002
). This organelle, which is morphologically and
functionally distinct from the early endosome, is a collection of tubular endosomes
concentrated near the centrosome. It is involved in the vesicle-mediated transport
of proteins and lipids that are endocytosed and then targeted either to the lysosome
or recycled back to the plasma membrane. For example, some cell surface pro-
teins, such as transferrin and low-density lipoprotein receptors, use this pathway:
they are first endocytosed, then separated from their respective ligands, and finally
recycled back to the cell surface via the ERC.
The placement of multiple distinct trafficking organelles next to the centrosome
indicates that this particular localization may be advantageous for a mammalian cell.
One obvious benefit is the convergence of trafficking compartments with the
microtubule network, which facilitates efficient vesicle-mediated transport to the cell
center and the cell periphery. In addition, the close proximity of the Golgi, the
ERGIC, and the ERC facilitates the rapid exchange of cargo among these com-
partments. For example, when TGN resident proteins, such as TGN38, escape from
the Golgi to the cell surface, they are recycled back to the TGN via the ERC (Ghosh
et al.
1998
). This trafficking route is also used by bacterial toxins (Mallard et al.
1998
). For instance, Shiga toxin is taken up by endocytosis, but avoids lysosomal
destruction by trafficking from the ERC to the TGN, and finally to the ER, where it is
released into the cytoplasm to inhibit ribosomal activity. Interestingly, under con-
ditions when normal recycling pathways are blocked, transferrin receptor travels
from the ERC to the pericentrosomal ERGIC and then back to the plasma membrane
(Marie et al.
2009
). Thus, the pericentrosomal region of the cell appears to serve as a
trafficking ''hub'' that promotes efficient cargo transfer between organelles.
While seen in some eukaryotes, including mammals, frogs, fish, and the unicel-
lular amoeba Dictyostelium (Distel et al.
2010
; Rehberg et al.
2005
; Reilein et al.
2003
; Thyberg and Moskalewski
1999
), a pericentrosomal Golgi ribbon is not
universal (Wilson et al.
2011
). In fact, Golgi membranes of many lower eukaryotes
are organized as isolated, unconnected membrane stacks, or individual cisternae
distributed throughout the cytoplasm. These Golgi membranes are fully functional
