Fig. 6.

Models for the 10-nm particle internal organization. Here, prestin molecules in the membrane are schematically represented by circles. Consider two kinds of bonds. One is strong, such as disulfide bond, which is involved in dimerization (shown in red), and the other is weak and responsible for alignment (green). a A model in which strong bonds are formed between the identical sites. This model leads to a linear polymer rather than tetramers because bonds (open and filled green squares) formed between two dimers should also form with the next dimers on the right and left. b A tetramer model in which strong bonds are formed between nonidentical sites 0 and 3, which are 90° apart. The residues exposed to the right and left of the tetramer, symbolically represented by open and closed green squares, are the same as those in the up-down direction. This results in a square lattice, contradicting preferential alignment in one direction. c A tetramer model with strong dimerizing bonds between sites 0 and 4, which are 120° apart. Yet another types of strong bonds are between site 1 and 3. The residues exposed to the right and left of the tetramer, symbolically represented by open and closed green squares, are different from those in the up-down direction. This tetramer can have a preferential direction of alignment, consistent with our observation, and also a symmetry compatible with hexagonal packing