Figure 5.

Cartwheel-dependent and -independent mechanisms for establishing the ninefold symmetry in the centriole. (a) Cartwheel and triplet microtubules arranged in ninefold symmetry in the normal centriole. (b) Abnormal cartwheels and microtubule arrangement in centrioles comprising truncated Bld10p. In bld10 cells expressing a Bld10p without the N-terminal 54% or the C-terminal 35%, various deficiencies are observed. For example, centrioles frequently have only eight triplets, the spoke and its tip are shorter and thinner, and the spoke tips are often detached from the triplets [26]. (c) Abnormal centrioles formed in the absence of cartwheels in bld12 cells. The majority of the centrioles are split in pieces of one to five triplet microtubules. The rest of the centrioles retain the circular arrangement, but have a variable number of triplets, ranging from seven to 11 [27]. (d) Speculations about cartwheel-independent mechanisms that produce ninefold symmetry. (i) If the linker connecting the A- and C-tubule of the triplets specifies the angle between the two triplet blades, nine microtubules would be on the centriole circle. (ii) Similarly, the linker connecting the singlet microtubules, which appears at an early stage of centriole assembly process [16], could specify the angle. (iii) An amorphous ring that appears before the formation of the cartwheel and restrict the loci for microtubule formation, may roughly determine the triplet number. (Online version in colour.)