Partially systematic molecular packing in the hexagonal columnar phase of dogfish egg case collagen

The collagen that forms the egg case of the dogfish Scyliorhinus canicula is stored in bulk in the female nidamental glands. Here the collagen molecules are thought to undergo a series of distinct pH-dependent liquid crystalline aggregation phase changes before assembling into the final arrangement encountered in the mature egg case. One liquid crystalline phase is hexagonal with the centres of two adjacent hexagons about 36 nm apart. We have collected tilt series of the hexagonal phase from plastic sections of the nidamental gland and have produced a three-dimensional reconstruction of the collagen arrangement of this phase. The reconstruction features axial columns of protein density lying regularly on the vertices of hexagonal cells of edge length 21 nm. Each column is connected to three nearest neighbours by irregular sheets of protein, but there appear to be preferred molecular directions at about 40 degrees to 50 degrees to the columns. The reconstruction has been interpreted in terms of known interactions of this collagen in other assemblies.