in the hexamer is probabilistic,
and not held to a particular firing
order as prescribed by concerted
or sequential models (Figure 2).
As the authors point out, this
mechanism is also well suited to
the biological properties of the
system. When an unfolded
polypeptide chain is translocated
through the hexameric ClpX ring,
each segment of the substrate is
conformationally and chemically
unique and may be located
anywhere in the ring. At any given
time, one ClpX subunit may be
better positioned than another to
interact with the substrate. As the
enzyme need not follow a
specified firing order, the subunit
best positioned to interact with
the substrate can hydrolyze ATP,
driving that particular round of
unfolding and translocation.
Is the probabilistic model for
AAA+ ATPase motor function
used by other proteins? Many
AAA+ machines act on
heterogeneous substrates,
suggesting that this mechanism
would be advantageous for other
members of the family. In
addition, mechanisms that invoke
a specified firing order for motor
function require specific, ordered
interactions between subunits.
The probabilistic mechanism is
not bound by such constraints
and can function similarly with the
wide array of quaternary
structures adopted by diverse
AAA+ enzymes. When appropriate
linkers can be designed,
comparable experiments with
other AAA+ ATPase machines will
determine whether the
probabilistic model is indeed a
general mechanism and will
provide new insights into how
these remarkable machines
function.
References
1. Ogura, T., and Wilkinson, A.J. (2001).
AAA+ superfamily ATPases: common
structure­diverse function. Genes Cells
6, 575­597.
2. Pickart, C.M., and Cohen, R.E. (2004).
Proteasomes and their kin: proteases in
the machine age. Nat. Rev. Mol. Cell
Biol. 5, 177­187.
3. Sauer, R.T., Bolon, D.N., Burton, B.M.,
Burton, R.E., Flynn, J.M., Grant, R.A.,
Hersch, G.L., Joshi, S.A., Kenniston, J.A.,
Levchenko, I., et al. (2004). Sculpting the
proteome with AAA(+) proteases and
disassembly machines. Cell 119, 9­18.
4. May, A.P., Whiteheart, S.W., and Weis,
W.I. (2001). Unraveling the mechanism of
the vesicle transport ATPase NSF, the N-
ethylmaleimide-sensitive factor. J. Biol.
Chem. 276, 21991­21994.
5. Caruthers, J.M., and McKay, D.B. (2002).
Helicase structure and mechanism. Curr.
Opin. Struct. Biol. 12, 123­133.
6. Hanson, P.I., and Whiteheart, S.W.
(2005). AAA+ proteins: have engine, will
work. Nat. Rev. Mol. Cell Biol. 6,
519­529.
7. Martin, A., Baker, T.A., and Sauer, R.T.
(2005). Rebuilt AAA + motors reveal
operating principles for ATP-fuelled
machines. Nature 437, 1115­1120.
8. Flynn, J.M., Neher, S.B., Kim, Y.I., Sauer,
R.T., and Baker, T.A. (2003). Proteomic
discovery of cellular substrates of the
ClpXP protease reveals five classes of
ClpX-recognition signals. Mol. Cell 11,
671­683.
9. Singh, S.K., Grimaud, R., Hoskins, J.R.,
Wickner, S., and Maurizi, M.R. (2000).
Unfolding and internalization of proteins
by the ATP-dependent proteases ClpXP
and ClpAP. Proc. Natl. Acad. Sci. USA
97, 8898­8903.
10. Kenniston, J.A., Baker, T.A., Fernandez,
J.M., and Sauer, R.T. (2003). Linkage
between ATP consumption and
mechanical unfolding during the protein
processing reactions of an AAA+
degradation machine. Cell 114, 511­520.
11. Gai, D., Zhao, R., Li, D., Finkielstein, C.V.,
and Chen, X.S. (2004). Mechanisms of
conformational change for a replicative
hexameric helicase of SV40 large tumor
antigen. Cell 119, 47­60.
12. Wang, J., Song, J.J., Seong, I.S.,
Franklin, M.C., Kamtekar, S., Eom, S.H.,
and Chung, C.H. (2001). Nucleotide-
dependent conformational changes in a
protease-associated ATPase HsIU.
Structure 9, 1107­1116.
13. Singleton, M.R., Sawaya, M.R.,
Ellenberger, T., and Wigley, D.B. (2000).
Crystal structure of T7 gene 4 ring
helicase indicates a mechanism for
sequential hydrolysis of nucleotides. Cell
101, 589­600.
14. Hersch, G.L., Burton, R.E., Bolon, D.N.,
Baker, T.A., and Sauer, R.T. (2005).
Asymmetric interactions of ATP with the
AAA+ ClpX6 unfoldase: allosteric control
of a protein machine. Cell 121,
1017­1027.
15. Joshi, S.A., Hersch, G.L., Baker, T.A.,
and Sauer, R.T. (2004). Communication
between ClpX and ClpP during substrate
processing and degradation. Nat. Struct.
Mol. Biol. 11, 404­411.
303 S. Frear, Department of
Biochemistry and Molecular Biology,
The Pennsylvania State University,
University Park, Pennsylvania 16802,
USA.
DOI: 10.1016/j.cub.2006.01.006
Current Biology Vol 16 No 2
R48
Matthew Collett1 and
Thomas S. Collett2
Almost every year new
discoveries increase one's
appreciation of the behavioural
sophistication of ants and bees,
making one wonder how the
cognitive capacities of these
small-brained animals measure up
to those of much larger-brained
mammals. Studying cognitive
capacities is particularly
informative within a behavioural
domain, such as navigation,
where different species do
roughly similar things. A paper by
Wehner and colleagues [1],
published recently in Current
Biology, introduces an interesting
new method for asking how
flexibly ants use landmark
memories when navigating within
familiar terrain.
Habits often mask behavioural
flexibility. On our habitual route to
work. we tend to perform, as if in
a trance, a sequence of
stereotyped actions that are often
cued by landmarks along the
route. Should we be stopped for
directions mid-route, then we may
wake up and, as we formulate a
reply, become aware of the many
types of spatial memories that we
have at our disposal, but which
are normally masked while we
follow our route. By pointing in the
direction of the requested
location, we can communicate its
position relative to where we are.
Or we can give a sequence of
instructions that describe a route
to the location, possibly choosing
between several routes. Such
route instructions, moreover, are
Insect Navigation: No Map at the
End of the Trail?
Although the hunt for cognitive maps in insects may not have reached
the end, the search itself has been fruitful in sharpening our
understanding of the ways that insects navigate through familiar
surroundings.
likely to include a description of
prominent landscape features
which serve as sign-posts or
beacons along the way. The
flexibility that allows us to imagine
a variety of possible routes to an
arbitrarily chosen goal or to
figure-out its direction, we take as
evidence for having what is
loosely called a cognitive map [2]
which endows landmarks with
positional information.
Insects such as ants and
honeybees also exhibit a
remarkable variety of navigational
memories. But are these
memories organised to allow a
similar degree of flexibility as that
available to humans? Ants
travelling from their nest to a
foraging site establish
idiosyncratic and stereotyped
routes [3]. Many species form
routes by laying pheromone trails,
but the routes of desert ants
depend upon the acquisition of
visual memories [4]. Landscape
features remembered as
landmarks can act as beacons,
guidelines or sign-posts that
trigger memories of subsequent
route-segments.
Ever since Gould [5] raised the
question in the mid 1980s, it has
been asked repeatedly whether an
insect's habitual route-following
might mask a richer
representation of familiar terrain in
which landmarks provide
positional information that could
support more flexible feats of
navigation [6­8]. This question is
often posed in terms of a forager's
ability to take a novel shortcut to a
familiar goal. For an experiment to
give a useful answer, one must
have evidence that at least three
conditions are met. First, the
release site must be familiar.
Second, foragers must be
motivated to travel towards the
goal -- either a food-site or nest.
Third, guidance should not be
available from landmarks near the
goal or along a habitual route to
the goal.
An exemplary study by Dyer [9]
to test whether food-motivated
foragers are able to take novel
shortcuts to a familiar food-site
illustrates the difficulties of doing
such experiments. Two groups of
honeybees were trained to one of
two feeder locations -- one higher
than the other -- in a slightly hilly
landscape (Figure 1). Bees trained
to one location were captured on
leaving the hive and released at
their non-trained location to see
whether they flew along the
shortcut to their trained feeder.
The choice of release sites
ensured an asymmetry in the
visual cues available to the
released bees. From the high
release site the bees could look
down on landmarks surrounding
their low-lying trained feeder.
While a minority followed the
compass cues from the hive or
took a path back to the hive, the
majority did indeed take the
shortcut.
In contrast, from the low-lying
release site, bees could not see the
landmarks surrounding the high-
location feeder to which they had
been trained. These foragers did
not take the shortcut: the majority
flew in the compass direction they
would have taken from the nest
(showing that they retained a food-
ward motivation), while a sizeable
minority took the route back to the
hive (showing that they recognised
the landscape, but used landmark
features in a home-motivated
fashion). With pre-training to the
low-lying feeder location all bees
followed route-cues back to the
hive. Foragers were thus able to
use familiar landmarks flexibly (in
the sense of recognizing them from
unexpected locations or
motivational states) but only as
indicators of a particular goal or
route. But might the bees'
propensity to go home have
masked something more map-like?
In their new displacement
experiment on the Australian ant,
Melophorus, Wehner et al. [1]
made sure that the ants were
familiar with landmarks at a
release site, while reducing as far
as possible the chances that the
ants' habit of following routes
could mask the use of any
positional memories that the ants
might have. Melophorus inhabits
an environment with abundant
grass tussocks and occasional
trees that can serve as
landmarks. Two long, low barriers
placed in this terrain led the ants
to follow a one-way foraging
circuit between their nest and a
feeder with a 2­5 m separation
between the food-ward and
homeward paths (Figure 2A).
The ants apparently established
idiosyncratic routes through this
terrain to reach the feeder. This
point is important, as consistent
routes, which differ between
individuals, provide the evidence
that foragers have indeed learnt
landmark features for use in
navigation. Ants returning from
the feeder were captured close to
the nest and released at a point in
the middle of the food-ward route.
Normally, the ants encountered
this release site only when they
were motivated to go to the
feeder. What would the ants do
when they viewed familiar
landmarks at the release site in a
Dispatch
R49
Figure 1. Novel shortcuts
require a view of the goal.
Honeybees trained to a low-
lying feeder (Low) were cap-
tured on leaving the hive and
released at a higher location
(High). The bees could look
down on landmarks sur-
rounding their trained feeder,
and the majority of the van-
ishing bearings (from High)
were towards their trained
feeder (Low). The vanishing
bearings in the converse
experiment (from Low), in
which the released bees
could not see their trained
feeder, were split between
the direction they would
have taken had they not
been displaced (Hive to
High) and the return direction
to the hive (Low to Hive).
(Adapted from [9].)
Current Biology
Hive
50 n
N
High
Low
265
262
259
homeward motivational state?
Might this unusual situation
uncover hidden positional
memories?
Displaced ants on release
performed a search that was
partly biased by landmarks, and
which led some of the ants to
encounter their homeward route.
The majority of these ants then
joined the route and, as would be
predicted [10], followed the route
cues home. Most significant is
what the ants failed to do while
searching, as these failures
indicate memories that the ants
either have not acquired or do
not use.
The first notable thing that the
ants did not do was to take a
direct path back to the nest
(Figure 2B). One kind of memory
that would have allowed them to
take this shortcut is a landmark-
associated path integration
memory. Path integration is the
process by which ants and bees
keep track of their position with
respect to their start point at the
nest -- not by means of
landmarks, but by integrating their
path as indicated by their sky
compass and some kind of
odometer [11]. Using the results of
its path integration, an ant or bee
can return from any position it has
reached in a direct line to its nest
without knowledge of the terrain it
covers. Insects could make their
navigation more versatile by
storing path integration states at
prominent locations and
associating these path integration
memories with landmark
memories [12]. The recall of a
path integration state when an
animal is displaced to a familiar
site could then allow the animal to
produce a direct path to any
location with stored path
integration coordinates.
That Melophorus did not take
the shortcut when displaced to a
location on their familiar one way
circuit [1] is consistent with
experiments on another desert
ant, Cataglyphis fortis. These
experiments tested whether
familiar landscape features along
a route can reset the state of an
ant's path integrator. There was
no sign of resetting either on the
way to the feeder [13], at the
feeder [14], at prominent
landmarks on the way home
[14,15], or at the nest [16]. Rather
than being used in association
with landmark memories, path
integration appears to be reserved
as an independent source of
information about a forager's
position based only on movement
information from the current trip.
The second notable thing that
the ants did not do was to retrace
their food-ward route back to the
nest (Figure 2B), suggesting that
ants have not incidentally learnt
the route in reverse. That the ants
took neither the reverse route nor
the shortcut home suggests in
addition that the ants have not
incorporated the landmark
memories into some kind of
cognitive map [1] or general
landscape memory [8]. In general
the question of whether insects
such as ants or honeybees have
cognitive maps is hard to settle
definitively. First, there is no
general agreement about what
constitutes a cognitive map [17].
Second, it is not always
straightforward to decide whether
experimental results can be safely
accepted as evidence for or
against map-like representations.
It is hard to refute the possibility
that apparently `map-like'
behaviour [18] can be explained
by direct visual guidance through
cues belonging to a learnt route
[9]. Equally, it is difficult to
discount the possibilities that
insects showing no map-like
behaviour either have too little
experience to acquire positional
memories (individuals may take
longer to acquire maps than
routes), or too much route
experience to use them. While one
could argue that in Wehner et al.'s
experiment [1] the ants had not
explored the interior of the
foraging circuit and so had no
opportunity to incorporate the
landmarks there into a general
landscape memory, the same
argument could not be applied to
the ants' failure to reverse their
familiar food-ward route. Instead,
the ants seem to limit the use of
landmark memories to
recognising goals or triggering
procedural instructions for
following routes [3,4,10,14].
The third notable thing that the
homebound ants mostly failed to
do was to follow the food-ward
route (Figure 2B). In contrast,
when these ants hit the homeward
route, the majority joined it
immediately and followed it all the
way to the nest. In this case, the
Current Biology Vol 16 No 2
R50
Figure 2. The ants' one-way
circuit.
Low barriers (thick lines)
encouraged ants to follow
different paths to the feeder
and on the way home.
(A) Ants caught on their way
home (C) when close to the
nest (N) were released (R)
on their path to the food (F).
(B) Three possible routes to
the nest that the ants failed
to take.
N
F
R
1 m
C
N
R
1
2
3
A
B
Food-ward
Homeward
Current Biology
Dispatch
R51
Lianna E. Swanson and
Greg J. Beitel*
Size does matter. While endlessly
debated in social circles, this is
unquestionably true for the normal
function of the epithelial and
endothelial tubes that comprise
such organs as the vascular
system, lung and kidney. The
enlarged tubules in a polycystic
kidney, for instance, literally crush
the surrounding normal tubules,
while narrowed blood vessels can
cause ischemic tissue injuries.
Surprisingly, the molecular and
cellular mechanisms of tube-size
regulation are poorly understood,
and therapeutics to intervene in
conditions such as polycystic
kidney disease are non-existent.
However, a flurry of papers,
including those by Luschnig et al.
[1] and Wang et al. [2] in this issue
of Current Biology, have now
defined a new mechanistic
framework for understanding
epithelial tube-size regulation in
one of the best studied models for
tubulogenesis, the Drosophila
tracheal system [3]. Importantly,
the combined results have
implications for understanding not
only tube-size control, but also
the mechanisms of specialized
apical secretion, the role of
extracellular matrix (ECM) in
controlling cell shape, and
possibly conserved signaling or
morphogenic roles for what has
been considered an invertebrate
specific oligosaccharide.
The stage was set for the Wang
and Luschnig papers [1,2] by
recent papers from the
Samakovlis, Uv, Krasnow,
Casanova and Nüsslein-Volhard
groups that showed that the
genetically programmed tripling of
the diameter of the tracheal tubes
requires the formation of a
transient lumenal chitin-based
ants' motivational state
determines so strongly which
landmarks the ants use that most
home-motivated ants ignore the
food-ward route entirely. In a
study with the two routes closer
together, displaced home-
motivated ants had the
opportunity to choose between
the routes; they then joined their
homeward route in preference to
the food-ward one [10]. That such
a preference can be triggered by
motivational state is borne out by
experiments on wood ants
showing that visual memories for
the ant's homeward or food-ward
route can be primed selectively
simply according to whether or
not the ant has fed [19].
The segregation of memory-use
according to motivational state
makes good sense. Often food-
ward and homeward routes are
intertwined so that an insect that
did not prime memories according
to motivational state might well
retrieve the memory for the wrong
route and so be guided in the
wrong direction. Ants seem not to
be misled in this way. Similarly, on
our way to work we may pass the
pub that we will visit on our way
home. In our hurry we may or may
not notice the pub, but we will
certainly not stop and go in. In
humans, such motivational
priming will probably influence
whether we notice a landmark, the
recognition of the landmark, as
well as the triggering of any
actions associated with the
landmark. In this respect, insects
and humans may not greatly
differ. Nonetheless, big brains
undoubtedly have their uses, even
for navigation.
References
1. Wehner, R. Boyer, M., Loertscher, F.,
Sommer, S., and Menzi, U. (2006). Ant
navigation: One-way routes rather than
maps. Curr. Biol. 16, January 10 issue.
2. Tolman, E.C. (1948). Cognitive maps in
rats and man. Psychol. Rev. 55, 189­208.
3. Wehner, R., Michel, B., and Antonsen, P.
(1996). Visual navigation in insects:
coupling of egocentric and geocentric
information. J. Exp. Biol. 199, 129­140.
4. Collett, T.S., Dillmann, E., Giger, A., and
Wehner, R. (1992). Visual landmarks and
route following in desert ants. J. Comp.
Physiol. [A] 170, 435­442.
5. Gould, J.L. (1986). The locale map of
honeybees: do insects have cognitive
maps? Science 232, 861­863.
6. Wehner, R., and Menzel, R. (1990). Do
insects have cognitive maps? Annu. Rev.
Neurosci. 13, 403­414.
7. Giurfa, M., and Capaldi, E.A. (1999).
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Neurosci. 22, 237­242.
8. Menzel, R., Brandt, R., Gumbert, A.,
Komischke, B., and Kunze, J. (2000).
Two spatial memories for honeybee
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based memories but not cognitive maps
in a familiar landscape. Anim. Behav. 41,
239­246.
10. Kohler, M., and Wehner, R. (2005).
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desert ants, Melophorus bagoti: how do
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11. Wehner, R., and Srinivasan, M.V. (2003).
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landmarks reset the global path
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14. Collett, M., Collett, T.S., Bisch, S., and
Wehner, R. (1998). Local and global
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15. Sassi, S., and Wehner, R. (1997). Dead
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16. Knaden, M., and Wehner, R. (2005). Nest
mark orientation in desert ants
Cataglyphis: what does it do to the path
integrator? Anim. Behav. 70, 1349­1354.
17. Bennett, A.T.D. (1996). Do animals have
cognitive maps? J. Exp. Biol. 199,
219­224.
18. Menzel, R., Greggers, U., Smith, A.,
Berger, S., Brandt, R., Brunke, S.,
Bundrock, G., Hulse, S., Plumpe, T.,
Schaupp, F., et al. (2005). Honeybees
navigate according to a map-like spatial
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19. Harris, R.A., Hempel de Ibarra, N.,
Graham, P., and Collett, T.S. (2005). Ant
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memories. Nature 438, 302.
1Department of Zoology, University of
Oxford, South Parks Road, Oxford OX1
3PS, UK. 2School of Life Sciences,
University of Sussex, Brighton BN1
9QG, UK.
E-mail: T.S.Collett@sussex.ac.uk.
DOI: 10.1016/j.cub.2006.01.007
Tubulogenesis: An Inside Job
New work shows that a dynamic and highly patterned apical
extracellular matrix regulates epithelial cell shape and tube size from
within the lumen of the Drosophila tracheal system.