Magnetosome Alignment

Well in a previous entry I wrote:

Generally, prokaryotes are devoid of membrane-bound organelles (including the nucleus, mitochondria, the endoplasmic reticulum), and their cytoskeletal systems are quite simple.


And I got a comment about magnetosomes, small membrane bound organelles found in certain bacteria that contain magnetite crystals. These organelles are usually aligned in long chains that orient along the earth's magnetic field and help these critters to swim north or south. (Some examples of magnetosomes are shown on the left).

This comment (and my quote) reminded me of this great paper in Science from earlier this year where researchers from Caltech describe how MamK, a homologue of the cytoskeletal proteins MreB and actin, forms long filaments that act to line up magnetosomes. To my knowledge this is the first example of the how the bacterial cytoskeleton is used to organize intracellular organelles.

Ref: Arash Komeili, Zhuo Li, Dianne K. Newman, Grant J. Jensen. Magnetosomes Are Cell Membrane Invaginations Organized by the Actin-Like Protein MamK Science (2006) 311:242 - 245

P.S. For the record I must add that other bacteria have organelles. The most famous are the stacked photosynthetic membranes found in Purple bacteria, such as Rhodospirillum rubrum and Rhodopseudomonas spheroides, and in Cyanobacteria.

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Well this is a very funny reference since I know the two senior authors on this paper, my colleagues. These magnetesomes are very cool but they are actually invaginations of the plasma membrane. Does that qualify them as organelles? I guess I don't know what the actual definition of an organelle is. When I was in high school I remember learning that ribosomes were organelles. Grant Jensen is doing a lot of other amazing things identifying substructure in bacteria. He's pushing tomography to the limit, tre cool.

Bil, glad you could join the conversation,

Does that qualify them as organelles?

Yes I saw that amazing figure in the paper (of a magnetosome bud). As to your comment - I don't see why not. Many membrane-bound organelles are derived from other membranous structures.

You haven't convinced me. Magnetosomes don't appear to ever separate fully from the periplasm. They are clearly organized and have structure, still...

Magnetosomes don't appear to ever separate fully from the periplasm.

Are you sure about that? Also your colleagues (and others) call them organelles. In the end it's all semantics ...

Generally, prokaryotes are devoid of membrane-bound organelles (including the nucleus, mitochondria, the endoplasmic reticulum), and their cytoskeletal systems are quite simple.
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When I was in high school I remember learning that ribosomes were organelles.

That combination of statements confused me. A ribosome isn't an organelle? Prokaryotes would have to have ribosomes, wouldn't they? How would you replace them? Are they not membrane-bound organelles? Should I assume by membrane-bound you mean that the organelle itself has a membrane and ribosomes don't?

By Norman Doering (not verified) on 09 Mar 2006 #permalink

Ribosomes are not membrane-bound, they are huge complexes of protein + ribosomal RNA. Some have claimed that ribosomes and other large structures (like centrioles) are organelles, hence the modifier "membrane-bound".

Are they organelles? It depends on your definition.