Unveiling the Intriguing Role of FAP20 in Tubulin Recruitment at the Doublet Microtubule Inner Junctions

In this issue of Structure, Bangera et al. investigate the role of the inner junction protein FAP20 in doublet microtubule assembly. Using cryo-EM and microtubule dynamic assays, they demonstrate that FAP20 recruits free tubulins to existing microtubule lattices, shedding light on B-tubule closure during doublet microtubule formation. In this issue of Structure, Bangera et al. investigate the role of the inner junction protein FAP20 in doublet microtubule assembly. Using cryo-EM and microtubule dynamic assays, they demonstrate that FAP20 recruits free tubulins to existing microtubule lattices, shedding light on B-tubule closure during doublet microtubule formation. Cilia and flagella are conserved hair-like organelles on the surface of eukaryotic cells that drive cell motility and fluid flow, sense environmental cues, and respond to extracellular stimuli. The cytoskeleton of cilia, known as the axoneme, comprises doublet microtubules, central apparatus microtubules, and associated proteins. Doublet microtubules are composed of α- and β-tubulin heterodimers and consist of a complete 13 protofilament A-tubule attached to a partial 10 protofilament B-tubule. The B-tubule and the A-tubule are bridged at the so-called “inner junction”, which is a non-tubulin protofilament composed of flagellar-associated protein 20 (FAP20 or CFAP20) and Parkin co-regulated gene protein (PACRG).1Yanagisawa H.a. Mathis G. Oda T. Hirono M. Richey E.A. Ishikawa H. Marshall W.F. Kikkawa M. Qin H. FAP20 is an inner junction protein of doublet microtubules essential for both the planar asymmetrical waveform and stability of flagella in Chlamydomonas.Mol. Biol. Cell. 2014; 25: 1472-1483Crossref PubMed Scopus (49) Google Scholar As the integral protein at the inner junction, it is expected that FAP20 serves an important role. Genetic knockout of FAP20 in Chlamydomonas reinhardtii results in motility deficiencies characterized by an abnormal and symmetrical waveform.1Yanagisawa H.a. Mathis G. Oda T. Hirono M. Richey E.A. Ishikawa H. Marshall W.F. Kikkawa M. Qin H. FAP20 is an inner junction protein of doublet microtubules essential for both the planar asymmetrical waveform and stability of flagella in Chlamydomonas.Mol. Biol. Cell. 2014; 25: 1472-1483Crossref PubMed Scopus (49) Google Scholar Morpholino-mediated knockdown experiments using zebrafish embryos revealed several phenotypes associated with ciliary dysfunction, including a curved body axis and shortened somite length.1Yanagisawa H.a. Mathis G. Oda T. Hirono M. Richey E.A. Ishikawa H. Marshall W.F. Kikkawa M. Qin H. FAP20 is an inner junction protein of doublet microtubules essential for both the planar asymmetrical waveform and stability of flagella in Chlamydomonas.Mol. Biol. Cell. 2014; 25: 1472-1483Crossref PubMed Scopus (49) Google Scholar FAP20 is also found at the inner junction of the doublet microtubule in primary cilia, which are non-motile cilia with extracellular sensory and signaling functions. In Caenorhabditis elegans, loss of CFAP20 causes a shortened B-tubule region2Chen Z. Li M. Zhu H. Ou G. Modulation of inner junction proteins contributes to axoneme differentiation.Proc. Natl. Acad. Sci. USA. 2023; 120e2303955120Google Scholar and disturbs sensory-dependent signaling and development.3Chrystal P.W. Lambacher N.J. Doucette L.P. Bellingham J. Schiff E.R. Noel N.C.L. Li C. Tsiropoulou S. Casey G.A. Zhai Y. et al.The inner junction protein CFAP20 functions in motile and non-motile cilia and is critical for vision.Nat. Commun. 2022; 13: 6595Crossref Scopus (4) Google Scholar Thus, CFAP20 contributes to axoneme differentiation and functions in both motile and non-motile cilia. Genetic studies in human patients link CFAP20 mutations to both primary ciliary dyskinesia and retinal dystrophy.3Chrystal P.W. Lambacher N.J. Doucette L.P. Bellingham J. Schiff E.R. Noel N.C.L. Li C. Tsiropoulou S. Casey G.A. Zhai Y. et al.The inner junction protein CFAP20 functions in motile and non-motile cilia and is critical for vision.Nat. Commun. 2022; 13: 6595Crossref Scopus (4) Google Scholar Cryogenic electron microscopy (cryo-EM) of axonemal structures revealed FAP20 bridges the A- and B-tubules at the inner junction of doublet microtubules and binds to central apparatus microtubules at a distinct interface (Figures 1A and 1B).4Khalifa A.A.Z. Ichikawa M. Dai D. Kubo S. Black C.S. Peri K. McAlear T.S. Veyron S. Yang S.K. Vargas J. et al.The inner junction complex of the cilia is an interaction hub that involves tubulin post-translational modifications.Elife. 2020; 9e52760Crossref Scopus (38) Google Scholar,5Gui M. Wang X. Dutcher S.K. Brown A. Zhang R. Ciliary central apparatus structure reveals mechanisms of microtubule patterning.Nat. Struct. Mol. Biol. 2022; 29: 483-492Crossref PubMed Scopus (17) Google Scholar Interestingly, FAP20 is highly expressed in ciliated tissues6Rijkers T. Rüther U. Sequence and expression pattern of an evolutionarily conserved transcript identified by gene trapping.Biochim. Biophys. Acta. 1996; 1307: 294-300Crossref PubMed Scopus (8) Google Scholar along with many non-ciliated tissues.4Khalifa A.A.Z. Ichikawa M. Dai D. Kubo S. Black C.S. Peri K. McAlear T.S. Veyron S. Yang S.K. Vargas J. et al.The inner junction complex of the cilia is an interaction hub that involves tubulin post-translational modifications.Elife. 2020; 9e52760Crossref Scopus (38) Google Scholar All these works point to the fact that FAP20 is a multifunctional protein that can interact with microtubules at different interfaces and acts as a structural nexus in both motile and non-motile cilia. This leads to many questions regarding the role of FAP20 in the assembly of cilia. In this issue of Structure, Bangera et al. present a functional and structural investigation of recombinant FAP20 from C. reinhardtii.7Bangera M. Dungdung A. Prabhu S. Sirajuddin M. Doublet microtubule inner junction protein FAP20 recruits tubulin to the microtubule lattice.Structure. 2023; 31: 1-10Google Scholar FAP20 binds to microtubules in a nucleotide-independent manner, as shown by biochemical pelleting assays. Using TIRF microscopy, FAP20 was shown to recruit free tubulins to the microtubule, suggesting that FAP20 acts as a linker between the microtubule lattice and free tubulin dimers by promoting their association with the growing microtubule. Moreover, FAP20 significantly reduces the frequency of catastrophe events. The increase in growth rate, length, and stability of microtubules in the presence of FAP20 highlights the crucial role of this protein in the formation and maintenance of microtubule structures in cilia. To understand FAP20-mediated tubulin recruitment to the microtubule lattice, Bangera et al. performed cryo-EM and AlphaFold2-assisted structure modeling on an in vitro reconstituted complex of microtubules, FAP20, and free tubulins.7Bangera M. Dungdung A. Prabhu S. Sirajuddin M. Doublet microtubule inner junction protein FAP20 recruits tubulin to the microtubule lattice.Structure. 2023; 31: 1-10Google Scholar They reported two remarkable observations. Firstly, in the presence of GTP, FAP20 nucleated the polymerization of protofilaments that were loosely stacked atop each other and formed a flexible cylinder. In demonstrating that FAP20 is sensitive to the conditions of microtubule reconstitution, it illustrates how the different interactions of FAP20 with doublet microtubules and central apparatus microtubules must be modulated during cilia assembly. Secondly, in the absence of GTP, microtubules were decorated with tubulin-bound FAP20 arranged in a continuous spiral and with a periodicity of approximately 11 nm along the microtubule axis (Figure 1C). It is possible that this unexpected spiral structure is due to free tubulins that modulate the microtubule binding of FAP20. The interactions between tubulin dimers, FAP20, and microtubules are flexible, resulting in rings/spirals at various angles relative to the microtubule axis. The authors suggest FAP20 mediates flexible interactions between the microtubule and tubulin dimers possibly by undergoing conformational changes or oligomerization. This observation of FAP20 mirrors other microtubule-associated proteins that recruit tubulin heterodimers to the microtubule lattice such as the tip-tracking protein Ska18Monda J.K. Whitney I.P. Tarasovetc E.V. Wilson-Kubalek E. Milligan R.A. Grishchuk E.L. Cheeseman I.M. Microtubule Tip Tracking by the Spindle and Kinetochore Protein Ska1 Requires Diverse Tubulin-Interacting Surfaces.Curr. Biol. 2017; 27: 3666-3675.e6Abstract Full Text Full Text PDF PubMed Scopus (22) Google Scholar or the motor protein kinesin-13 that binds to both straight and curved microtubules.9Mulder A.M. Glavis-Bloom A. Moores C.A. Wagenbach M. Carragher B. Wordeman L. Milligan R.A. A new model for binding of kinesin 13 to curved microtubule protofilaments.J. Cell Biol. 2009; 185: 51-57Crossref PubMed Scopus (36) Google Scholar Bangera et al. have shown that the binding of FAP20 to microtubules and recruitment of free tubulin represent a dynamic and intermediate step during the formation of the doublet microtubule.7Bangera M. Dungdung A. Prabhu S. Sirajuddin M. Doublet microtubule inner junction protein FAP20 recruits tubulin to the microtubule lattice.Structure. 2023; 31: 1-10Google Scholar Their model for inner junction assembly proposes that FAP20, PACRG, and MEIG1 (depending upon the organism) are recruited to the inner junction. Only those proteins that bind to both the microtubule and the B-tubule protofilament are stably associated. As the inner junction protofilament in doublet microtubules, FAP20 and PACRG alternate every 8 nm.4Khalifa A.A.Z. Ichikawa M. Dai D. Kubo S. Black C.S. Peri K. McAlear T.S. Veyron S. Yang S.K. Vargas J. et al.The inner junction complex of the cilia is an interaction hub that involves tubulin post-translational modifications.Elife. 2020; 9e52760Crossref Scopus (38) Google Scholar,5Gui M. Wang X. Dutcher S.K. Brown A. Zhang R. Ciliary central apparatus structure reveals mechanisms of microtubule patterning.Nat. Struct. Mol. Biol. 2022; 29: 483-492Crossref PubMed Scopus (17) Google Scholar Though PACRG is stoichiometrically equivalent to FAP20 as part of the inner junction protofilament, it is unclear whether PACRG is an immediate necessity. Genetic deletion of PACRG does not disrupt primary cilia in C. elegans.2Chen Z. Li M. Zhu H. Ou G. Modulation of inner junction proteins contributes to axoneme differentiation.Proc. Natl. Acad. Sci. USA. 2023; 120e2303955120Google Scholar One of the questions that comes out of this work concerns FAP20 regulation. The observation that FAP20 nucleates the polymerization of tubulin protofilaments atop the microtubule lattice in the presence of GTP begs the question of how FAP20 activity is regulated in cilia. Another question that arises from this investigation relates to the microtubule-binding interface. At the inner junction of the doublet microtubules, FAP20, sandwiched by two PACRG molecules, binds to α-tubulins of the A1 protofilament and the intra-dimer interface between the α- and β-tubulin heterodimer of the B10 protofilament at the inner junction.4Khalifa A.A.Z. Ichikawa M. Dai D. Kubo S. Black C.S. Peri K. McAlear T.S. Veyron S. Yang S.K. Vargas J. et al.The inner junction complex of the cilia is an interaction hub that involves tubulin post-translational modifications.Elife. 2020; 9e52760Crossref Scopus (38) Google Scholar,5Gui M. Wang X. Dutcher S.K. Brown A. Zhang R. Ciliary central apparatus structure reveals mechanisms of microtubule patterning.Nat. Struct. Mol. Biol. 2022; 29: 483-492Crossref PubMed Scopus (17) Google Scholar Indeed, Bangera et al. also found that FAP20 bound to the surface of α-tubulin of in vitro polymerized microtubules exclusively.7Bangera M. Dungdung A. Prabhu S. Sirajuddin M. Doublet microtubule inner junction protein FAP20 recruits tubulin to the microtubule lattice.Structure. 2023; 31: 1-10Google Scholar Yet in the central apparatus, FAP20 forms distinctive interactions by binding to the surface of the C2 singlet microtubule through its associations with FAP65 and FAP147 rather than the microtubule lattice.5Gui M. Wang X. Dutcher S.K. Brown A. Zhang R. Ciliary central apparatus structure reveals mechanisms of microtubule patterning.Nat. Struct. Mol. Biol. 2022; 29: 483-492Crossref PubMed Scopus (17) Google Scholar Astonishingly, FAP20 is positioned atop β-tubulin as opposed to α-tubulin.5Gui M. Wang X. Dutcher S.K. Brown A. Zhang R. Ciliary central apparatus structure reveals mechanisms of microtubule patterning.Nat. Struct. Mol. Biol. 2022; 29: 483-492Crossref PubMed Scopus (17) Google Scholar The work done by Bangera et al. serves as a template to investigate how FAP20 binds uniquely to singlet microtubules when additional FAP proteins are present.7Bangera M. Dungdung A. Prabhu S. Sirajuddin M. Doublet microtubule inner junction protein FAP20 recruits tubulin to the microtubule lattice.Structure. 2023; 31: 1-10Google Scholar Using biochemical and structural biology, and computational approaches, Bangera et al. provided insights into the role of FAP20 during doublet microtubule assembly. An intriguing observation lies in FAP20’s sensitivity to various microtubule types and nucleotide states, suggesting the possibility of dynamic variations in assembly states. The authors propose that FAP20 is recruited to the inner junction and stabilizes the doublet microtubule by binding to both the A1 and B10 protofilaments. K.H.B. is funded by research grants from the Natural Sciences and Engineering Research Council of Canada and Canadian Institute of Health Research. The authors declare no competing interests.