Abstract
Discovering mechanisms governing organelle assembly is a fundamental pursuit in biology. The centriole is an evolutionarily conserved organelle with a signature 9-fold symmetrical chiral arrangement of microtubules imparted onto the cilium it templates. The first structure in nascent centrioles is a cartwheel, which comprises stacked 9-fold symmetrical SAS-6 ring polymers emerging orthogonal to a surface surrounding each resident centriole. The mechanisms through which SAS-6 polymerization ensures centriole organelle architecture remain elusive. We deploy photothermally-actuated off-resonance tapping high-speed atomic force microscopy to decipher surface SAS-6 self-assembly mechanisms. We show that the surface shifts the reaction equilibrium by ~104 compared to solution. Moreover, coarse-grained molecular dynamics and atomic force microscopy reveal that the surface converts the inherent helical propensity of SAS-6 polymers into 9-fold rings with residual asymmetry, which may guide ring stacking and impart chiral features to centrioles and cilia. Overall, our work reveals fundamental design principles governing centriole assembly.
Highlights
Discovering mechanisms governing organelle assembly is a fundamental pursuit in biology
How do molecular interactions that occur at the nanometer length scale translate into the stereotyped architecture of the nascent centriole at the organelle scale, including its orthogonal emergence from a surface surrounding the resident centrioles and its characteristic chirality? Our work provides initial elements of answer to these questions
By analyzing the self-assembly of the SAS-6 ring polymer at the single molecule level, we found that ring assembly is drastically favored by the surface, where the polymerization reaction follows a coagulation-fragmentation process
Summary
Discovering mechanisms governing organelle assembly is a fundamental pursuit in biology. The first structure in nascent centrioles is a cartwheel, which comprises stacked 9-fold symmetrical SAS-6 ring polymers emerging orthogonal to a surface surrounding each resident centriole. HsSAS-6 is recruited to a toroidal surface that surrounds the resident centriole, and which contains the interacting proteins Cep152/ Cep63/Cep[57] Because STIL binds HsSAS-6, such focusing could direct HsSAS-6 recruitment and assembly to a single location, ensuring formation of only one nascent centriole per resident centriole Upon recruitment to the torus, SAS-6 proteins are thought to somehow form ring polymers and stack in the vertical direction, orthogonal to the resident centriole, thereby generating a structural scaffold for the nascent centriole organelle A HsSAS-6 mutant defective in ring polymer formation localizes throughout the torus[16], indicating that focusing of SAS-6 proteins requires their polymerization
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