Ls. Van Kirk et al., Ultrastructure of Rickettsia rickettsii actin tails and localization of cytoskeletal proteins, INFEC IMMUN, 68(8), 2000, pp. 4706-4713
Actin-based motility (ABM) is a mechanism for intercellular spread that is
utilized by vaccinia virus and the invasive bacteria within the genera Rick
ettsia, Listeria, and Shigella. Within the Rickettsia, ABM is confined to m
embers of the spotted fever group (SFG), such as Rickettsia rickettsii, the
agent of Rocky Mountain spotted fever. Infection by each agent induces the
polymerization of host cell actin to form the typical F (filamentous)actin
comet tail. Assembly of the actin tail propels the pathogen through the ho
st cytosol and into cell membrane protrusions that can be engulfed by neigh
boring cells, initiating a new infectious cycle. Little is known about the
structure and morphogenesis of the Rickettsia rickettsii actin tail relativ
e to Shigella and Listeria actin tails. In this study we examined the ultra
structure of the rickettsial actin tail by confocal, scanning electron, and
transmission electron microscopy. Confocal microscopy of rhodamine phalloi
din-stained infected Vero cells revealed the typhus group rickettsiae, Rick
ettsia prowazekii and Rickettsia typhi, to have no actin tails and short (s
imilar to 1- to 3-mu m) straight or hooked actin tails, respectively. The S
FG rickettsia, R. rickettsii, displayed long actin tails (>10 mu m) that we
re frequently comprised of multiple, distinct actin bundles, wrapping aroun
d each other in a helical fashion. Transmission electron microscopy, in con
junction with myosin S1 subfragment decoration, revealed that the individua
l actin filaments of R. rickettsii tails are >1 mu m long, arranged roughly
parallel to one another, and oriented with the fast-growing barbed end tow
ards the rickettsial pole. Scanning electron microscopy of intracellular ri
ckettsiae demonstrated R. rickettsii to have polar associations of cytoskel
etal material and R. prowazekii to be devoid of cytoskeletal interactions.
By indirect immunofluorescence, both R. rickettsii and Listeria monocytogen
es actin tails were shown to contain the cytoskeletal proteins vasodilator-
stimulated phosphoprotein profilin, vinculin, and filamin. However, rickett
sial tails lacked ezrin, paxillin, and tropomyosin, proteins that were asso
ciated with actin tails of cytosolic or protrusion-bound Listeria. The uniq
ue ultrastructural and compositional characteristics of the R. rickettsii a
ctin tail suggest that rickettsial ABM is mechanistically different from pr
eviously described microbial ABM systems.