Biology Reference
In-Depth Information
Flask-shaped caveolae have been shown to pinch off from the
plasma membrane in a dynamin-dependent manner.
27,28
Caveolar
endocytosis does not occur constitutively, but is rather triggered by
the cargo, e.g. simian virus 40 (SV40; Ref. 29). Caveolae structures
have been shown to accumulate in the cytoplasm and fuse with pre-
existing caveosomes. The best markers for the caveolae are their major
structural proteins, caveolin-1 and caveolin-2, which make a rope-like
coating on the cytoplasmic side of these structures. Caveolin-1 is the
best molecular marker for the caveosomes, too. The other character-
istics of the caveosomes are the cytoplasmic location and the absence
of classical endosomal markers. The morphological characteristics of
caveosomes are based on cellular structures where SV40 is internal-
ized, and it is possible that there are also other types of caveosomes.
The presence of clathrin- and caveolin-independent endocytosis
pathways has been suggested on the basis of studies using cells lack-
ing morphologically typical caveolae structures or studies in which the
clathrin and caveolae pathways have been inhibited.
25,30
Similarly, some
of these entry pathways are dependent on dynamin, but dynamin-
independent pathways may also exist.
31
Recently, increasing attention has been paid to the macropinocy-
tosis pathway.
30
It is usually elicited from the raft domains on the
plasma membrane. It involves ruffling of plasma membrane, which
may take place rapidly after the binding of the ligand to its receptor.
This ruffling seems to be dependent on Rho GTPases. Actin drives
the formation of membrane ruffles, which collapse on the plasma
membrane and are engulfed into the cell with a relatively large vol-
ume of liquid. It was recently suggested that carboxy-terminal bind-
ing protein 3/brefeldin A-ribosylated substrate (CtBP3/BARS)
regulates macropinocytic uptake in a similar manner as dynamin reg-
ulates clathrin- and caveolae-dependent pathways.
31
CtBP3/BARS
was earlier shown to mediate the fission of tubules on the Golgi.
32
Already our first confocal microscopy studies indicated that EV1
and
integrin do not enter the host cells via the classical endoso-
mal structures. We found only marginal colocalization of EV1 with
transferrin and no apparent colocalization with EEA1, CI-MPR or
CD63, markers of early and late endosomes/lysosomes.
10
In the same
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