![Structures of microtubule plus ends and the associated caps and plugs in centrioles and cilia. (A) Scheme of the growing plus end of a microtubule, characterized by flared protofilaments of varying lengths, and a longer or shorter stabilizing GTP cap (light gray tubulin). (B) Illustration of a mother centriole–daughter centriole pair surrounded by PCM. The fully elongated mother centriole is ∼0.5 μm in length and has triplet microtubules arranged in radial symmetry in the proximal two thirds. A triplet has an A-tubule with 13 protofilaments, a B-tubule, and a C-tubule with 10 protofilaments each. The C-tubule terminates growing, and thus, the distal one third is composed of doublet microtubules (just A- and B-tubules). The centriolar cap complex (green) is present on the distal end of both mother and daughter centrioles. (C) Illustration of motile and primary cilia. The basal body, composed of triplet microtubules, acts as a template for the axoneme, which is the core of the cilium. The axoneme has doublet microtubules arranged in radial symmetry (9 + 0 arrangement) and surrounded by the ciliary membrane. Motile cilia have an additional pair of singlet microtubules in the center called the central pair (9 + 2 arrangement). The doublet microtubules transition into singlet microtubules in the distal ciliary compartment. The ciliary tip complex (green) is present on the distal tip of the ciliary microtubules. (D) Tomogram slice and corresponding 3D model depicting the flared protofilaments of a microtubule plus end in the dorsal root ganglion neurons, from Foster et al. (2022). (E) Tomograms and corresponding 3D models depicting the variable structure of flared plus ends of microtubules grown in vitro. Modified from McIntosh et al. (2018). (F) Tomogram slice and the corresponding diagram highlighting what is shown in the tomogram, illustrating filamentous proteins extending into the central pair microtubules in cilia of Tetrahymena thermophila. Modified from Legal et al. (2023). (G) Tomogram and associated 3D model showing the electron-dense cap structure found on the plus end of microtubules grown in vitro in the presence of centriolar proteins (from Iyer et al. [2025]). (H) Cork-like electron-dense structures found on the plus end of microtubules grown in the presence of ciliary proteins in vitro (from Saunders et al. [2025]). The plus ends in G and H look relatively blunt compared to the control microtubules in E. Refer to the image caption for details.](https://cdn.rupress.org/rup/content_public/journal/jcb/225/6/10.1083_jcb.202601174/2/jcb_202601174_fig1.png?Expires=1782934940&Signature=XOwcbGTzNwnBmYonZ6HvCJkCyJbI84rhQOpg6wZ6UR8gcHe0CT3WyWTBS7ZJcZXC5ufR~1dn1nuTjQi-lOB1snrnaS51tF5PsGyA9tHtRWrZT2h4Xd1~uA29Mv5JWjjrioVuYbnqPr~KieGijXJAU3CrAZtftTcYWu-2NOJ53PcXWHnoukvPiduaJ3O08T9WpwxcrqkGnpK7GVc2Zoy7uoO7bd6di7T8YQ6No7CHK9J6brt~2M7XdogCpPUVr-yc1lrFu1TwdlF0Z~zELz4YU6oc6t5pCV7vQRomg6n2TYCVN3~~n4V2wWhNkl-hneCV7fJFI0k3Rmd2LkyoaRmdZw__&Key-Pair-Id=APKAIE5G5CRDK6RD3PGA)
Panel A shows schematic diagram of microtubule plus-end growth illustrating GTP cap length and protofilament structure. Panel B shows schematic diagram of centriole structure highlighting mother–daughter centrioles, microtubule triplets, and distal appendages. Panel C shows schematic diagram comparing motile and primary cilia architecture with axoneme arrangements and basal body structure. Panel D shows electron tomography image and 3D model of microtubule plus ends, highlighting flared protofilaments. Panel E shows electron tomography images with 3D models illustrating variability in microtubule plus-end structures in vitro. Panel F shows electron tomography images highlighting outwardly curling protofilaments in growing centriole A-tubules. Panel G shows electron tomography image and 3D model illustrating electron-dense cap structures at microtubule plus ends. Panel H shows electron tomography images of sperm microtubule end structures, highlighting luminal plug features. Panel I shows electron tomography image and models illustrating cork-like cap structures at microtubule plus ends.
S tructures of microtubule plus ends and the associated caps and plugs in centrioles and cilia. (A) Scheme of the growing plus end of a microtubule, characterized by flared protofilaments of varying lengths, and a longer or shorter stabilizing GTP cap (light gray tubulin). (B) Illustration of a mother centriole–daughter centriole pair surrounded by PCM. The fully elongated mother centriole is ∼0.5 μm in length and has triplet microtubules arranged in radial symmetry in the proximal two thirds. A triplet has an A-tubule with 13 protofilaments, a B-tubule, and a C-tubule with 10 protofilaments each. The C-tubule terminates growing, and thus, the distal one third is composed of doublet microtubules (just A- and B-tubules). The centriolar cap complex (green) is present on the distal end of both mother and daughter centrioles. (C) Illustration of motile and primary cilia. The basal body, composed of triplet microtubules, acts as a template for the axoneme, which is the core of the cilium. The axoneme has doublet microtubules arranged in radial symmetry (9 + 0 arrangement) and surrounded by the ciliary membrane. Motile cilia have an additional pair of singlet microtubules in the center called the central pair (9 + 2 arrangement). The doublet microtubules transition into singlet microtubules in the distal ciliary compartment. The ciliary tip complex (green) is present on the distal tip of the ciliary microtubules. (D) Tomogram slice and corresponding 3D model depicting the flared protofilaments of a microtubule plus end in the dorsal root ganglion neurons, from Foster et al. (2022). (E) Tomograms and corresponding 3D models depicting the variable structure of flared plus ends of microtubules grown in vitro. Modified from McIntosh et al. (2018). (F) Tomogram slice and the corresponding diagram highlighting what is shown in the tomogram, illustrating filamentous proteins extending into the central pair microtubules in cilia of Tetrahymena thermophila. Modified from Legal et al. (2023). (G) Tomogram and associated 3D model showing the electron-dense cap structure found on the plus end of microtubules grown in vitro in the presence of centriolar proteins (from Iyer et al. [2025]). (H) Cork-like electron-dense structures found on the plus end of microtubules grown in the presence of ciliary proteins in vitro (from Saunders et al. [2025]). The plus ends in G and H look relatively blunt compared to the control microtubules in E.