Re: How do organelles move inside the cell and where does the energy come from?

Date: Tue Jan 30 01:15:34 2001
Posted By: Alex Goddard, Grad student, Neuroscience, Harvard Medical School
Area of science: Cell Biology
ID: 980386618.Cb

Message:

Nathan,
Good question! The most important thing to consider with your question is perspective – who’s doing the movement? Indeed, cells require mitochondria to generate the energy needed to move themselves around or perform complex motions like peristalsis. The simple fact is that organelles don’t have sources of energy to move themselves about – but the cells can move them around. Organelles are best viewed as structural components of the cell, like an elevator in a building; they aren’t mini-cells within the cells (although one could argue this about mitochondria – but that’s a whole different story!)
Your specific questions about exactly how an organelle moves or how a protein gets through the ER and Golgi are two functionally different processes. They fall under the rubric of two processes: organelle movement and protein trafficking. I’m not sure if you want the nit-gritty details, but they are below if you want them!

First, organelle movement. I’ll consider big things like mitochondria or other membranous bodies. As I mentioned before, organelles don’t have their own source of energy. They utilize the ATP generated in the mitochondria to get around. But to really understand how the organelles move, we need to probe a bit deeper to ask, How are they using that energy?

You started to get at this with one of your questions – the one about whether a chemical attraction does the work. And of course, the annoying, scientist’s answer is always: yes, kinda. The organelles need to have something to move around on – they just can’t propel themselves around the cytoplasm. There exists an elaborate, internal structural network throughout the entire cell – not unlike the girders that support a building. (picture to the left, from Molecular Probes) This "cytoskeleton" is the surface that organelles can move on. Among the proteins that make up the cytoskeleton, microtubules are the predominant "walkway" organelles use to move around the cell. Specialized ‘motor proteins’ are attached to both the organelle and the microtubules (via chemical interactions). These proteins, called kinesins (which generally move towards the periphery) and dyneins (which move towards the center) ‘walk’ the organelle up and down the microtubules. It’s these motor proteins that utilize ATP to move around, and drag the organelle with it. (Follow this link for a moving cartoon illustrating how kinesin might "walk" on a microtubule)

Next, Protein trafficking! For whatever reason, the processes used to move vesicles from the ER and through the Golgi are different from those used to move organelles to the various parts of the cell. But I should note that once a set of proteins has moved through the Golgi, and is ready to go to its final destination, the vesicle which contains those proteins may utilize the aforementioned microtubule system!
To travel forward through the Golgi, microtubules are not needed. (If a protein is a little messed up, and needs to go to the previous ‘pouch’ of the Golgi, that does require microtubules). What mediates most of the forward flow is mixture of proteins working together. More specifically, some proteins are involved in pinching off a bud on the ER and Golgi membrane, and another subset mediate which membrane the newly formed vesicle will fuse with. Proteins called coatomers form a cluster around a bit of membrane. As more cluster together, they tend to pull out a piece of membrane and eventually cause a vesicle to pinch off.
Specialized proteins called SNAREs mediate the directionality of the process, making sure things don’t move backwards when they shouldn’t. SNAREs are both on the vesicle and the target membrane, but not the 'sending' membrane. v-SNAREs on the vesicles recognize the t-SNARE on the target membrane and allow fusion. But the lack of the appropriate t-SNARE on the sending membrane prevents the newly formed vesicle from just fusing backwards.
Both of these processes do require energy. GTP (the ATP analog) is used to allow SNAREs to recognize each other and allow for various interactions of the coatomers.

Cells use both of these processes to get a lot done, and even use variations on the themes to achieve novel, complex behaviors. For example, the release of neurotransmitter at the synapse requires protein synthesis and transport of those proteins to the axon terminal (the ending which releases transmitter – it’s usually a good distance away from the cell body). Neurotransmitter is also stored in vesicles at the terminal, and these vesicles have SNAREs to mediate the fusion with the cell membrane – the event which causes transmitter release. But these aren’t your average, ordinary SNAREs – they are specially designed to release neurotransmitter ONLY when the right signal comes along (as opposed to the Golgi system, in which any vesicle with the right SNARE will fuse as soon as it finds the right target).

I hope this of some use! I got most of this info from the Bible of molecular biology, The Molecular Biology of the Cell, by Bruce Alberts and Co. (3^rd edition, 1994, Garland Publishing)

Alex G
cgoddard@fas.harvard.edu

Some good websites are: http://www.cellbio.com/
and http://www.accessexcellen ce.org/index.html
and http://esg- www.mit.edu:8001/esgbio/7001main.html (the MIT online Bio book)
They have lots of great links, images, and other lil’ nuggets of knowledge.

A primer on cytoskeleton can be found at http://www.biology.arizona.edu/cell_bio/tutorials/cytoskeleton/main.ht ml

Current Queue | Current Queue for Cell Biology | Cell Biology archives

Try the links in the MadSci Library for more information on Cell Biology.