Here is another short video as I try to figure out whether the models proposed for surfactant protein A (and D) and some other collectins actually fit what is seen with electron microscopy.  This video is not the best for several reasons of which the main reason is the densities within the tubular organization that are so frequently posted in the literature are not consistent from report to report.  This is to be expected with the different kinds of fixatives, post-fixatives, stains and sectioning procedures, so that is not a surprise. There are sets of images where at least 9 different densities are seen within sets of individual rectangles of tubular myelin (see videos on this site), and some times when only the four corners are dense, and of course there are all variations in between.

The point of this video was to take the actual surfactant protein A, as a proportionally-sized molecule, put it together in the 18-mer and see whether it really does fit (as the flower bundle shown by so many authors) into the four corners of the tubular myelin grid.  The answer was: not great, the main difficulty being that the helical neck region where it connects to the collagen-like portion of the molecule doesn’t really sync up in the real electron micrographs like it does in the diagrams.  (see this video for samples of trying to fit the 18-mer into each of the four corners of sampled tubular myelin grid squares. It just isn’t all that convincing…. but it should be.  I make this one comment.  There is an article which is by Hansen and Holmskov (1998) which actually shadow casts surfactant A and D and shows images.  They comment on the rigid structure of surfactant protein D in shadow cast images, but I am contrasting that with the floppy-ness of surfactant protein A in their shadow cast images.

I would submit that the surfactant D molecules hold their cross like rigor, as depicted in diagrams, where their shadow cast surfactant A certainly does not look like the bouquet of flowers that most diagrams make it out to be.  I am thinking that the floppy arrangement of the shadow cast molecules (pictures of surfactant A which show the 6 bundles of trimers falling randomly all over the place like a loosely held together bunch of balls on strings, is perhaps more reflective of the real situation and that the organization of surfactant A in the corners of tubular myelin is not as tidy as has been deemed.  Video below is produced from composites of rectangles from online images of tubular myelin (in this case i believe the series of images is from a large chunk of tubular myelin within a publication of Hassett Engleman and Kuhn 1980 (without permission).

Surfactant protein A molecules are black and size adjusted to be a relative 20 nm within in one 100 nm rectangle of the tubular myelin. The tubular myelin profiles were randomly selected areas from an electron micrograph from above reference and modified per:  cropping,  rotating, skewing, contrast, brighness, and size.