Essential Cell Biology 5th edition

Microtubules
587
minus
end
tail
globular head
globular head
tail
cargo
microtubule
adaptor protein
adaptor protein
kinesins
plus
end
cytoplasmic
dynein
Figure 17–20 Different motor proteins
transport different types of cargo along
microtubules. The transport of cargo
toward the plus end of a microtubule is
carried out by different types of kinesin
motors, each of which is thought to
transport a specific set of vesicles,
organelles, or molecules. In some cases, the
tail of the kinesin binds directly to the cargo,
while in other cases, different adaptor
proteins allow the same type of kinesin to
carry different cargos. Transport toward
the minus end is mediated by cytoplasmic
dynein, which generally uses adaptor
proteins to interact with its selected cargo.
cargo (e.g., vesicle)
Microtubules and Motor Proteins Position Organelles in
the Cytoplasm
Microtubules and motor proteins play an important part in positioning
organelles within a eukaryotic cell. In most animal cells, for example, the
tubules of the endoplasmic reticulum (ER) reach almost to the edge of
the cell (Movie 17.5), whereas the Golgi apparatus is located in the cell
interior, near the centrosome (Figure 17–21A). The ER extends out from
its points of connection with
ECB5
the nuclear
n17.100/17.20
envelope along microtubules,
which reach from the centrally located centrosome out to the plasma
membrane. As a cell grows, kinesins attached to the outside of the ER
membrane (via adaptor proteins) pull the ER outward along microtubules,
stretching it like a net (Figure 17–21B). Cytoplasmic dyneins attached to
microtubule
Golgi
centrosome
nucleus
(A)
(B)
(C)
10 µm
Figure 17–21 Microtubules help position organelles in a eukaryotic cell. (A) Schematic diagram of a cell showing the typical
arrangement of cytoplasmic microtubules (green), endoplasmic reticulum (blue), and Golgi apparatus (yellow). The nucleus is shown
in brown, and the centrosome in light green. (B) One part of a cell in culture stained with antibodies to the endoplasmic reticulum
(blue, upper panel ) and to microtubules (green, lower panel ). Kinesin motor proteins pull the endoplasmic reticulum outward along the
microtubules. (C) A different cell in culture stained with antibodies to the Golgi apparatus (yellow, upper panel ) and to microtubules
(green, lower panel ). In this case, cytoplasmic dyneins pull the Golgi apparatus inward along the microtubules to its position near
the centrosome, which is not visible but is located on the Golgi side of the nucleus. (B, courtesy of Mark Terasaki, Lan Bo Chen, and
Keigi Fujiwara; C, courtesy of Viki Allan and Thomas Kreis.)