Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1286 Chapter 23: Pathogens and Infection
human
phagocyte
Legionella
bacteria
Legionella
blocking of
endosome
fusion
type IV secretion
system
ER vesicles
nucleus
Legionellacontaining
vacuole
ribosomes
(A)
0.5 µm
(B)
Figure 23–26 Legionella pneumophila residing in a compartment with characteristics similar to those of the rough
endoplasmic reticulum (ER). (A) Electron micrograph showing the unusual coiled structure that the Legionella pneumophila
bacterium induces on the surface of a phagocyte during the invasion process. Some other pathogens, including the bacterium
Borrelia burgdorferi, which causes Lyme disease, the eukaryotic pathogen Leishmania, and the yeast Candida albicans, can
also invade cells using this type of coiling phagocytosis. (B) Following invasion, L. pneumophila uses its type IV secretion system
to secrete effector proteins that block phagosome–endosome fusion and phagosome maturation. It also secretes effector
proteins that promote the fusion of the phagosome with ER-derived vesicles, thereby creating a Legionella-containing vacuole
with characteristics similar to the rough ER. (A, from M.A. Horwitz, Cell 36:27–33, 1984. With permission from Elsevier.)
MBoC6 m24.25/23.26
spread to humans by central air-conditioning systems, which harbor infected
amoebae and produce microdroplets of water that are easily inhaled. Once in
the lung, the bacteria are engulfed by macrophages by an unusual process called
coiling phagocytosis (Figure 23–26A). L. pneumophila uses a type IV secretion
system to inject effector proteins into the phagocyte that modulate the activity
of proteins that regulate vesicular traffic, including SNARE proteins and Rab and
Arf family small GTPases (discussed in Chapter 13). The effector proteins thereby
prevent the phagosome from fusing with endosomes and promote its fusion with
vesicles derived from the endoplasmic reticulum, converting the phagosome into
a compartment that resembles the rough endoplasmic reticulum (Figure 23–26B).
Viruses can also alter membrane traffic in the host cell. Enveloped viruses
make use of host cell membranes to acquire their own envelope membrane. In
the simplest cases, virally encoded glycoproteins are inserted into the endoplasmic
reticulum membrane and follow the secretory pathway through the Golgi
apparatus to the plasma membrane; the viral capsid proteins and genome assemble
into nucleocapsids, which acquire their envelope as they bud off from the
plasma membrane (see Figure 23–12). This mechanism is used by many enveloped
viruses including HIV-1. Other enveloped viruses such as herpesviruses and
vaccinia virus acquire their lipid envelopes in more complex ways (Figure 23–27).
Viruses and Bacteria Use the Host-Cell Cytoskeleton for
Intracellular Movement
As mentioned earlier, many pathogens escape into the cytosol rather than
remaining in a membrane-enclosed compartment. The cytosol of mammalian
cells is extremely viscous, as it is crowded with protein complexes, organelles, and
cytoskeletal filaments, all of which inhibit the diffusion of particles the size of a
bacterium or a viral nucleocapsid. Thus, to reach a particular region of the host
cell a pathogen must be actively moved there. As with transport of intracellular
organelles, pathogens generally use the host cell’s cytoskeleton for their active
movement.