Bi-directional shortening of muscle and the direction of eukaryotic cell crawling are both dependent on the polar nature of microfilaments or microtubules. It is ultimately the filament ploarity that determines the direction, either through biased polymerization or their motor proteins. Most myosins are (+) end directed, just as most kinesins are (-) end directed, and in vivo one typically sees preferential growth of actin filaments and microtubules from their (+) ends.
Can anyone think of a counter-example? Is there a system where the motor sets the ultimate direction of cell movement rather than the filament polarity? Perhaps a system where the filaments are of random polarity yet directional motion still occurs.
Not off-hand. However some cell types contain odd proteins that specify cell polarity and directional extension. You may be aware of this example, original paper URL below, that is now well studied.
A Unique Cytoskeleton Associated with Crawling in the Amoeboid ...
jcb.rupress.org/content/108/1/55.full.pdf
How about with flagella? They have a set microtubule polarity, yet cells can swim in many different ways.
Are you talking about active movement? I do not know very well but is it possible for any kind of non-adherent culture system? For example, chemo tactic movement of HL60?
Research at the Walter and Eliza institute (Melbourne, Australia), specifically within the lab of Dr. Jake Baum, showed that malaria parasites (P. falciprum) show irregular migration patterns at the sporozoite stage. The sporozoite exhibits a spiral-like motion around a fixed axis yet is capable of directional migration toward a target blood source. In this system, and as far as my current knowledge goes, they are uncertain of the motor that drives motility and that cytoskeletal filament (actin, microtubules and IFs) polarity has not be found to hold a major role in directing the force of motion.
Away from P. falciparum, has it not been shown that actin polymerisation can act perpendiular and/ or in multiple vectors away from the general polarised state of cytoskeletal filaments during directed migration in cancerous cell lines? The literature out there at present describes a transitional state between mesenchymal and amoeboid migration during cancer cell invasion into tissues that exhibit 3D matrices. During this transition I think the cell is driven forward not by polarisation of actin or microtubules, but rather propulsion caused by through flow of the cytoplasm. Sorry for the essay!
Dear William,
(i) The polarity of motor proteins you consider - as I could undwerstand from your message - in a linear manner.
However, there are cases when these motor proteins are working in a circular way, e.g. in plant cells the cyclosis which results the running around the chloroplasts is also up to motorproteins - but not in a linear way.
(ii) Please also consider that motor proteins are beating cearly in one direction: e.g. kinesins are walking to plus end in general but some derivatives are walking to the negative end.
Christopher: The Apicomplexa parasites (including P. falciparum) are exactly what I had im mind. The motor in this case *is* known (MyoA), but the mechanism of directionality is not. I'm wondering if there are any parallel examples.
Laszlo's example sounds interesting. I'll have to check it out. The more I learn about plants the cooler they get!
Dear William,
Apart from the Apicomplexa parasites there is a knowledge base out there on the gliding motility of bacteria leading to potential cell invasion. But other than that I cannot elaborate further, except Laszlo's idea is quite interesting because the plant cells do seem to exhibit similar motile patterns.
My only other suggestion would be searching for homologs/ paralogues etc. to what the Apicomplexa parasites use, but in other cell types. This may be achieved through online programmes for prediction of protein-protein interaction, or protein-protein threading to determine any structural similarities. Maybe the using the Aa sequence for TRAP family invasins may be a good place to start? I know its all superficial as its literally data mining, but it may provide a good starting platform. Just some ideas. I hope the search serves you well!
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