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A. Based on our quick-freeze deep-etch images of
F1F0 ATP synthases of mitochondrial inner
membranes (figures 17-21 of Chapter 2) and of proton translocating
V1V0 ATPases on the decorated tubules of the CVC
in P. multimicronucleatum, I have proposed a model for membrane
tubule formation in these two organelles. Beginning with single
F1F0 holoenzymes or single
V1V0 holoenzymes, that are each transmembrane
protein complexes, I envision that these holoenzymes start to nucleate
within the bilayer of the planar inner membrane of a mictochondrion or
within the bilayer of a vesicle near the CVC that is destined to form
a bundle of decorated tubules. Nucleation continues as a ribbon of two
or three rows of subunits, lying side-by-side, grow in length within
the plane of the membrane. Assuming a rigid lateral association of the
holoenzymes that produces a slight bend and twist in the ribbon as it
grows it will cause the bilayer to bulge out of the membrane plane.
Further lengthening will complete a helical turn and will pull the
bilayer of the planar membrane out into a tubule. Continued growth
will extend the tubule indefinitely. B. In the case of decorated tubules the tubule itself is straight as there are two separate helical bands wrapped around one tubule. Only two rows are shown forming one helical band, however, I now think the band is composed of triplets of dyads with the dyads lying side-by-side to form a “brick” and these “bricks” are linked side by side with other triplet dyads to form a helical band, one brick wide but of undefined length. Two such helical bands wrap around each tubule. C. In the inner membrane of the mitochondrion only one helical band of F1F0 units with a much shallower pitch wraps around the outer margin of each tubule causing the tubule itself to take on a helical shape. F1F0 holoenzymes do not form dyads but a helical band is formed of a long string of two F1F0 complexes lying side-by-side like a zipper. This drawing was published as Figure 5A-C in Allen, Protoplasma 189:1-8, 1995.
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