Figure 6. Working model of cartwheel assembly.

A single ring of SAS-6 (blue) serves as an initial seed onto which pairs of SAS-6 rings are recruited preferentially in a stepwise manner. Our work suggests that the D1 density (black) could arise from the C-terminus of the CrSAS-6 coiled coil, whereas the D2 density appears upon interaction with a fragment of Bld10p (orange). The amorphous ring and the initial microtubules recruited to the core cartwheel are represented as well; note that the amorphous ring could be present in vivo earlier than represented. Asterisks represent the capability of a SAS-6 ring to assemble with another one. In one hypothetical scenario, cartwheel growth could be terminated by the addition of a single SAS-6 ring, which, in contrast to the addition of double rings, would not endow the cartwheel with the capacity to further incorporate incoming SAS-6 rings. See text for further details.