[2005/01/01] Cellular UPS Gets Right Packages to Chloroplasts
Cellular UPS Gets Right Packages to Chloroplasts 01/01/2005 If all your
packages were sent correctly over the holidays, consider the job a plant
cell has getting 3000 proteins into a chloroplast. Mistakes are not
just inconvenient. They can be deadly, or at least bring
photosynthesis to a halt. To guarantee proper delivery of
components, plant cells have a remarkable shipping system, described in
Current Biology by two UK biologists, Paul Javis and Colin
Robinson.1 Part of the challenge is getting polypeptides
into the chloroplasts, which have a double membrane. A remarkable
crew of enzymes and molecular machines puts a shipping label (transit
peptide) on each amino acid chain, reads it, routes it to the correct
destination, and then removes it:
Over 90% of the ~3000 different proteins
present in mature chloroplasts are encoded on nuclear DNA and
translated in the cytosol [cell fluid outside the
nucleus]. These proteins are synthesized
in precursor form – each bearing an amino-terminal targeting
signal called a transit peptide – and are imported into the
organelle by an active, post-translational targeting process
(Figure 1). This process is mediated by molecular machines
in the outer and inner envelope membranes, referred to as ¡®translocon at
the outer envelope membrane of chloroplasts¡¯ (Toc) and
¡®translocon at the inner envelope membrane of chloroplasts¡¯
(Tic), respectively. Upon arrival in the stroma
[chloroplast interior], the transit peptide
is removed and the protein either takes on its final conformation or
is sorted to one of several internal compartments in a
separate targeting process. (Emphasis added in all
quotes.)
The authors believe, like most evolutionists, that
plastids (including chloroplasts) arose when a primordial cell engulfed
another and took over its light-harvesting machinery, a process called
endosymbiosis (see refutation by Don
Batten). They believe the former cell that became the
chloroplast retained only a stripped down version of its genetic code, and
most of the DNA instructions for building these 3000 chloroplast proteins
got transferred to the nucleus. Yet this means that a tremendous
amount of machinery had to be developed to get the proteins to their
destinations:
Chloroplasts are complex organelles
comprising six distinct suborganellar compartments: they have
three different membranes (the two envelope membranes and the
internal thylakoid membrane), and three discrete aqueous
compartments (the intermembrane space of the envelope, the stroma
and the thylakoid lumen). One of the consequences of this
structural intricacy is that the internal routing of
chloroplast proteins is a surprisingly complex process. While
envelope proteins may employ variations of the Toc/Tic import pathway to
arrive at their final destination, proteins destined for the thylakoid
membrane or lumen employ one of four distinct targeting pathways
(Figure 1). Thylakoid membrane proteins are targeted by the
signal recognition particle (SRP)-dependent and spontaneous
insertion pathways, whereas lumenal proteins are targeted by the Sec
and Tat pathways....
Each of these ¡°pathways¡± is an
assembly-line process involving multiple proteins dedicated to these
tasks. Several points brought out in the article make it challenging
to perceive of a smooth transition from endosymbiosis to today¡¯s complex
shipping and handling pathways (numbering ours):
The transit peptide needs to fit the receptor on the membrane, and
another protein has to be ready to cleave it (remove it).
The transit peptides have to be precise to avoid having the protein
arrive at the wrong organelle, like the endoplasmic reticulum,
mitochondrion or peroxisome – organelles which also accept polypeptides
with shipping labels.
Transit peptides are varied. ¡°One might therefore
expect chloroplast transit peptides to share well-defined
primary or secondary structural motifs,¡± they say. ¡°On the
contrary, transit peptides are remarkable in their
heterogeneity. They vary in length from 20 to >100
residues, and have no extended blocks of sequence conservation.¡±
The transit proteins ¡°do not seem to form secondary structure in
aqueous solution¡± but once they arrive at their target membrane, they
seem to take on a characteristic structure.
The polypeptides (precursor proteins) are threaded through the
needle of specialized gates in the membrane. There, additional
molecular machines (chaperones) make sure they do not fold prematurely.
To get a polypeptide through a membrane involves three steps:
contact, docking, and translocation, when the transit peptide is
cleaved. This requires energy: a high concentration of ATP must be
present for the operation.
The Toc and Tic squads, like a delivery organization with a variety
of employees skilled in particular tasks but working on common goals, is
made up of multiple proteins, each with their own task to perform, all
working in coordination.
Once inside the outer membrane, the polypeptide has to get past the
inner membrane. Another set of specialized proteins are available
for that task.
A third import apparatus has to complete the task of getting the
polypeptide to its final destination. Many go to the thylakoid
membrane, rich with light-harvesting structures and ATP synthase.
Those polypeptides bound for the thylakoid membrane have a secondary
shipping label (transit peptide). In addition, they may have a
¡°stop-transfer¡± signal to indicate their destination.
Removal of the secondary transit peptide can occur by ¡°one of two
very different pathways,¡± called Sec and Tat. Sec transports
proteins in an unfolded state, but Tat can transport them in a folded
state. Each pathway involves multiple proteins working together.
In the Tat pathway, ¡°There is even evidence that some proteins are
exported in an oligomeric form¡± [i.e., several proteins bound together
in a complex], ¡°which points to a remarkable translocation
mechanism,¡± they remark. Is this like squeezing a completed
sweater through the eye of a needle? ¡°...we currently know very
little about this mechanism,¡± they say. ¡°Somehow, this system must
transport a wide variety of globular proteins – some over 100 kDa
[kilodaltons] – while preserving the proton motive force and
avoiding loss of ions and metabolites.¡± Their surprise at
this indicates it is quite a feat.
The translocation process can expend 30,000 protons, ¡°a substantial
cost by any standard.¡± According to current theory, a pH
difference between inner and outer membrane provides the proton flow,
but that pH balance must be carefully monitored and regulated.
Another pathway named SRP inserts proteins into the lumen. The
authors claim this pathway was ¡°clearly inherited from the
cyanobacterial progenitor of the chloroplast,¡± but admit that there are
differences in the insertion pathways and events at the thylakoid
membrane in chloroplasts. ¡°...it is fair to state that, while the
major players in this pathway have been identified, their modes of
action remain unclear and we do not understand how such
highly hydrophobic proteins are bound by soluble factors,shuttled to the membrane and then handed over to
membrane apparatus and inserted.¡±
Evolutionists who expected the SRP pathway from E. coli
bacteria to act the same in chloroplasts, where homologous proteins were
detected, learned otherwise: ¡°Surprisingly, this is not the
case. In vitro assays for the insertion of a range of membrane
proteins have shown that the vast majority of such proteins do not
rely on any of the known protein transport machinery, including SRP,
FtsY, Alb3 or the Sec/Tat apparatus, for insertion.¡± Nor do they
rely on nucleoside triphosphates or proton flow.
Speaking of the apparent spontaneous insertion of the thylakoid
proteins, they comment, ¡°This unusual pathway for membrane protein
insertion appears to be unique to chloroplasts.¡± Though the
typical insertion components are not involved, they believe it would be
¡°overly simplistic¡± to assume that this pathway requires no ¡°complex
insertion apparatus.¡±
Other pathways than those described above are used for other
proteins to get inside the chloroplast. Some are encoded by the
chloroplast DNA, translated in the interior, then transported to their
destinations.
Chloroplasts have to transport not only the essential
light-harvesting proteins, but also ¡°housekeeping¡± proteins for
structural maintenance. These must be imported at their own
separate rates depending on the stage of development or the
environmental conditions, and have their own specific transit peptides.
This represents the state of our knowledge on protein transport
in chloroplasts. It is only a partial picture of a varied and
complicated picture with many players, as their final paragraph makes
clear:
The Tat pathway manages the remarkable
feat of transporting large, folded proteins without
collapsing the delta-pH, and we currently know very little about this
mechanism. Most membrane proteins use a possibly ¡®spontaneous¡¯
insertion mechanism that just does not make sense at the moment –
why do these proteins need so little assistance from translocation
apparatus, when membrane proteins in other organelles and
organisms need so much? And how do these thylakoid proteins
avoid inserting into the wrong membrane? We have gone
some way toward understanding the rationale for the
existence of all these pathways, but the thylakoid may
still have surprises in store.
1Paul Jarvis and Colin Robinson, ¡°Mechanisms of
Protein Import and Routing in Chloroplasts,¡± Current
Biology, Volume 14, Issue 24, 29 December 2004, Pages R1064-R1077,
doi:10.1016/j.cub.2004.11.049.
If you survived this long and
mind-numbing description of chloroplast protein transport, you have
gasped at the complexity of it all. On a general level, getting a
protein from one place to another sounds simple. But look how many
players are involved, how many checks and balances, how many protection
mechanisms and signals are required to get the packages delivered
accurately. And this is all just to get the chloroplast to start
to get ready to begin to commence doing its job: harvesting light for
photosynthesis (and that¡¯s another story: if this one was over your
head, run for cover). You saw these authors refer to
the Darwinian just-so story that once upon a time, a bacterium engulfed
a cyanobacterium that had learned how to harvest light, and the two
shared their technologies, forming a glorious partnership that led to
plants. For that to be true, all these new protein transporting
mechanisms had to be invented to get the genes moved into the nucleus,
and then to get their translated proteins back into the
chloroplasts. Why didn¡¯t the chloroplast DNA stay put? Did
they offer any plausible way an unpredictable series of accidents led to
all this complexity? Assuredly not. Considering the
difficulty in getting just one protein right by chance, it
strains credibility, light-years beyond the breaking point, to think
that these complexes of complicated proteins – all working like a
company – arose by chance. Nor would an unbiased person presume
that time would suffice for thousands of beneficial mutations to occur
for even one of the pathways to emerge, even assuming natural selection
– that magic wand of the Darwinists – preserved them. But
one component is useless if not part of a functioning system. That
is the power of the argument from irreducible complexity. We¡¯re
talking about three thousand proteins needing special delivery to
make a chloroplast work, and each one requires dozens if not
hundreds of amino acids in the correct sequence. Would one of the
Toc proteins have any selective value if the other components were not
present to help it get the polypeptide through the outer membrane?
Clearly not. Even if all the Toc proteins emerged somehow, and
managed to squeeze the polypeptide through the outer membrane, the
polypeptide would just sit there uselessly without the Tic proteins to
get it through the second membrane. If it got past the inner
membrane, it would be useless unless it folded correctly with the aid of
chaperones, and then made it to the exact destination in the
chloroplast, where, working together with other proteins, it could
perform its spectacular feat: converting light energy to chemical
energy. But all the players in the system need that energy
to do their jobs! It may be tedious to wade through
some of these articles about cellular mechanisms, but take the time once
in awhile, because the power of the the message – intelligent design –
is in the details. Hold this evidence up in the face of the Darwin
Party and ask them some hard-hitting questions: how could such
coordinated complexity arise by unguided, mindless, purposeless
processes? Did Fed Ex or UPS emerge from a tornado in a
junkyard? It¡¯s details like this that convinced Antony Flew, the
prominent (former) atheist, that the case for intelligent design was
compelling, and over many years, convinced him to become a theist. With some hard heads and hard hearts, overkill and
persistence is necessary.