Biology concepts – protein moonlighting, undulipodia,
evolution, basal body, centriole, GAPDH, intraflagellar transport
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Today’s
post is on a multitasking cell structure. This
would
make Alton Brown proud, since he hates tools
that
do only one thing. The University of Miami of
Florida
football team runs through fire extinguisher
blasts
when they enter the stadium – maybe Alton
can
find a second use for his.
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This phenomenon is called protein moonlighting. The re-evaluation of the human genome (about
19,000 genes) suggests that many proteins have more than one distinct function.
This would allow for a relatively small number of genes to provide a large
functional proteome (the total number of protein functions). As such, a 2014 study is showing the
importance of moonlighting proteins in health and biology.
There are rules for a protein to have a legitimate second
job. It’s only moonlighting if the two functions are unrelated, the
functions can't be carried out by two different domains of the protein either. This would
suggest a gene fusion event. The two functions must be independent, so ablating
one doesn’t affect the other.
There are hundreds of examples of moonlighting proteins in
the literature now, and more are sure to follow. There are examples aplenty within the
glycolysis pathway; you know, the breakdown of sugar for energy. No fewer than
seven of the ten glycolytic enzymes are known to have other jobs.
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Crystallin
proteins (alpha and beta) make up the
majority
of the lens and cornea of the eye. They are
transparent,
but they do more than that. Recent
studies
show that they have enzymatic activity in other
tissues.
Aldehyde dehydrogenase and transketolase are
enzymes
that turn out to be moonlight crystallins.
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But as amazing as GAPDH is, today’s example of a multitasker
is even more rare, in that the moonlighter is a complete structure, made of
many proteins, and has two distinctly different jobs. What’s more, each
function has its own exceptions. What's our structure of interest? The basal body, or perhaps I should call it
the centriole.
Let’s talk about the basal body first. This is the base of
the eukaryotic undulipodia (cilia
and flagella). These moving tails come in two parts; the basal body and the axoneme. We talked at length about the axoneme a few months ago, with its nine doublet microtubules surrounding two
singlet microtubules (9[2]+2, see picture below). Undulipodia movement, as
opposed to the motor driven prokaryotic flagella, is achieved by sliding the
different doublets forward and back past one another.
But in those previous posts we didn’t talk much about the
basal body. It too is a ring of microtubules, although these are shorter
polymers that in the axoneme. Instead of doublets, there are almost always nine
sets of triplet microtubules, and there are no center microtubules (9[3]+0). Of
course, there were a couple of exceptions, and we talked about them.
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The
basal body has a 9(3) + 0 structure, while the
axoneme
is 9(2) + 0. While this cartoon shows the
complexity
of the axoneme, our post today highlights
the
complexity of the basal body. One thing this cartoon
does
show, the axoneme is sheathed in the plasma
membrane,
it isn’t a protein structure sticking out through
the
membrane.
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The basal body serves as the nucleation site for
mictrotubule growth into the axoneme. It’s like a skyscraper, the microtubule
girders are built vertically on the basal body foundation; only here, the basal
body sparks a self-assembly of the microtubules. You don’t need fearless guys climbing the beams to build an axoneme.
If we turn our attention to the centriole, we find that it’s
used in mitosis. When a cell divides, each progeny cell receives one centrosome. Don’t confuse the
centrosome with a centriole or a centromere (a near center point of a
chromosome, it holds the two chromatids together). The centrosome is more of an
area, it contains two centrioles, a mother and a daughter, and the
pericentriolar matrix (PCM) amorphous group of proteins that help the
centrioles do their job.
Each centriole is a complex microtubule formation in a
9(3)+0 arrangement, about 120 nm wide and 175 nm long. This is exactly the
structure of the basal body – they’re the same thing! Well, almost.... a centriole
has to mature into a basal body.
During S phase of the cell cycle (when the chromosome are
replicated) the centrosome will duplicate. Each centriole grows another one
from its side, at a right angle. The mother is a mother again, and the daughter
becomes a mother for the first time. The two mother/daughter pairs then gather their own PCMs and
move to the sides of the nucleus. When mitosis time comes, microtubules grow
toward the chromsosomes, but from the mother centriole only. This is the
spindle and will help pull the chromatids apart during cell division.
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Start
at the top left. During the cycle, a pair of centrioles
will
separate and each will grow another from the proximal
end.
The daughter centrioles then have to mature, with
proteins
added toward the distal end. During mitosis, they
two
pairs separate and form the spindle body, which pulls
the
chromatids apart.
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A commentary published in 2010 talks about how many cell
types, including some oocytes can undergo meiosis without centrioles, while
centriole numbers than are re-established after fertilization. However, in other
tissues, loss of centrioles leads to genetic instability over time, even if the
spindle will develop without the centrosome. To this point, we still haven’t
resolved the issue of whether centrioles are necessary for proper cell
division.
In the vast majority of cases, centrioles come from
centrioles. One serves as a template to form the second. The process through
which this occurs has just begun to be revealed in the past few years. There is
a linking fiber from the proximal end of the mother centriole that acts as a
seed point to start aggregation of the daughter centriole at a 90˚ angle to the
first.
The process is very complicated, but is carried out
spontaneously, without specific gene products to guide it. It was first
believed that if centrioles were lost, then they could not be regained. Of
course, they also thought that centrioles had their own DNA. Now we know that in
cases where centrioles are lost, they can form de novo, and function just fine
in mitosis or as basal bodies. In several studies, removal of centrioles or
cells without centrioles to begin with allows for new centrioles being formed
from aggregated microtubules.
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Intraflagellar
transport is the method by which the
axoneme
grows, adding tubulin monomers to the end.
The
proteins are carried up the axoneme by walking
proteins.
These are important not for just cilia assembly,
but
also for cell signaling and sensing that occurs in
the
undulipodia.
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Finally, there is attachment of accessory bodies like
rootlets to anchor it to the membrane, transition fibers to move to the axoneme
and distal appendages. Only after all this maturation of the basal body can the
axoneme be built by IFT. This is how the process happens in all species EXCEPT fruit fly male
gametes and the microbe Plasmodium yoelii,
where the axoneme grows BEFORE plasma membrane docking of the basal body.
What’s more, you can always reverse it and go from basal
body back to centriole, so their functions must somewhat overlap. The
basal body and centriole both serve as microtubule
organizing centers (MTOCs). So what makes it a moonlighting structure?
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Prokaryotic
flagella just whip around in a circle, but
eukaryotic
undulipodia have more of a beating motion.
The
side view shows how the flow is one direction, while
the
top view indicates that the motion is circular, but not
like
a propeller.
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That’s a little weird, but what’s really weird is the kids. In a 1991 paper, part of the oviduct of a
quail was reversed, so that the cilia beat toward the ovary instead of toward the
uterus. When cells divided through embryo and chick development, the cilia of
the progeny cells had the same orientation as those in the parent! They continued to beat in the wrong direction. Since basal
bodies are duplicated from basal bodies, the mother basal bodies gave rise to
daughters that beat the same direction.
The evidence presented shows that we have an
ancient, yet complex, structure that has a couple of important jobs. The question for evolutionary biologists
is which came first, the basal body or the centriole? The last common
eukaryotic ancestor (LECA), the cell from which all eukaryotic cells descend,
had undulipodia, so the basal body is an extremely old structure. But all
eukaryotic cells undergo mitosis or meiosis, so the centriole must be important
too.
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Notice
the cilia that are the upper most in the animation.
They
show best the coordinated beating that pushes fluid
in
one direction.
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Cells without either centrioles and basal bodies suggest that basal body function came first; since you can’t have undulipodia without basal bodies, but our discussion above shows you can have mitosis without centrioles.
On the other hand, centrioles have to mature to become basal
bodies, wouldn’t this suggest that they came first? Or perhaps the basal body
was the primary product and nature managed to find a use for one if its
precursors. What do you think, is the basal body the chicken or the egg?
Next week let’s look at one of the animal kingdoms great
exceptions in terms of cilia, even though primitive and higher animals have
them, round worms don’t cilia or flagella!
For
more information or classroom activities, see:
Protein
moonlighting –
Centriole
–
Basal
body –
Centrosome
–
