The conserved Bardet-Biedl syndrome proteins assemble a coat that traffics membrane proteins to cilia.

The BBSome is a complex of Bardet-Biedl Syndrome (BBS) proteins that shares common structural elements with COPI, COPII, and clathrin coats. Here, we show that the BBSome constitutes a coat complex that sorts membrane proteins to primary cilia. The BBSome is the major effector of the Arf-like GTPase Arl6/BBS3, and the BBSome and GTP-bound Arl6 colocalize at ciliary punctae in an interdependent manner. Strikingly, Arl6(GTP)-mediated recruitment of the BBSome to synthetic liposomes produces distinct patches of polymerized coat apposed onto the lipid bilayer. Finally, the ciliary targeting signal of somatostatin receptor 3 needs to be directly recognized by the BBSome in order to mediate targeting of membrane proteins to cilia. Thus, we propose that trafficking of BBSome cargoes to cilia entails the coupling of BBSome coat polymerization to the recognition of sorting signals by the BBSome.

Short disordered protein segment regulates cross-species transmission of a yeast prion.

Soluble prion proteins contingently encounter foreign prion aggregates, leading to cross-species prion transmission. However, how its efficiency is regulated by structural fluctuation of the host soluble prion protein remains unsolved. In the present study, through the use of two distantly related yeast prion Sup35 proteins, we found that a specific conformation of a short disordered segment governs interspecies prion transmissibility. Using a multidisciplinary approach including high-resolution NMR and molecular dynamics simulation, we identified critical residues within this segment that allow interspecies prion transmission in vitro and in vivo, by locally altering dynamics and conformation of soluble prion proteins. Remarkably, subtle conformational differences caused by a methylene group between asparagine and glutamine sufficed to change the short segment structure and substantially modulate the cross-seeding activity. Thus, our findings uncover how conformational dynamics of the short segment in the host prion protein impacts cross-species prion transmission. More broadly, our study provides mechanistic insights into cross-seeding between heterologous proteins.

Molecular basis for diversification of yeast prion strain conformation.

Self-propagating ß-sheet-rich fibrillar protein aggregates, amyloid fibers, are often associated with cellular dysfunction and disease. Distinct amyloid conformations dictate different physiological consequences, such as cellular toxicity. However, the origin of the diversity of amyloid conformation remains unknown. Here, we suggest that altered conformational equilibrium in natively disordered monomeric proteins leads to the adaptation of alternate amyloid conformations that have different phenotypic effects. We performed a comprehensive high-resolution structural analysis of Sup35NM, an N-terminal fragment of the Sup35 yeast prion protein, and found that monomeric Sup35NM harbored latent local compact structures despite its overall disordered conformation. When the hidden local microstructures were relaxed by genetic mutations or solvent conditions, Sup35NM adopted a strikingly different amyloid conformation, which redirected chaperone-mediated fiber fragmentation and modulated prion strain phenotypes. Thus, dynamic conformational fluctuations in natively disordered monomeric proteins represent a posttranslational mechanism for diversification of aggregate structures and cellular phenotypes.

αTAT1 catalyses microtubule acetylation at clathrin-coated pits.

In most eukaryotic cells microtubules undergo post-translational modifications such as acetylation of α-tubulin on lysine 40, a widespread modification restricted to a subset of microtubules that turns over slowly. This subset of stable microtubules accumulates in cell protrusions and regulates cell polarization, migration and invasion. However, mechanisms restricting acetylation to these microtubules are unknown. Here we report that clathrin-coated pits (CCPs) control microtubule acetylation through a direct interaction of the α-tubulin acetyltransferase αTAT1 (refs 8, 9) with the clathrin adaptor AP2. We observe that about one-third of growing microtubule ends contact and pause at CCPs and that loss of CCPs decreases lysine 40 acetylation levels. We show that αTAT1 localizes to CCPs through a direct interaction with AP2 that is required for microtubule acetylation. In migrating cells, the polarized orientation of acetylated microtubules correlates with CCP accumulation at the leading edge, and interaction of αTAT1 with AP2 is required for directional migration. We conclude that microtubules contacting CCPs become acetylated by αTAT1. In migrating cells, this mechanism ensures the acetylation of microtubules oriented towards the leading edge, thus promoting directional cell locomotion and chemotaxis.

The major alpha-tubulin K40 acetyltransferase alphaTAT1 promotes rapid ciliogenesis and efficient mechanosensation.

Long-lived microtubules found in ciliary axonemes, neuronal processes, and migrating cells are marked by α-tubulin acetylation on lysine 40, a modification that takes place inside the microtubule lumen. The physiological importance of microtubule acetylation remains elusive. Here, we identify a BBSome-associated protein that we name αTAT1, with a highly specific α-tubulin K40 acetyltransferase activity and a catalytic preference for microtubules over free tubulin. In mammalian cells, the catalytic activity of αTAT1 is necessary and sufficient for α-tubulin K40 acetylation. Remarkably, αTAT1 is universally and exclusively conserved in ciliated organisms, and is required for the acetylation of axonemal microtubules and for the normal kinetics of primary cilium assembly. In Caenorhabditis elegans, microtubule acetylation is most prominent in touch receptor neurons (TRNs) and MEC-17, a homolog of αTAT1, and its paralog αTAT-2 are required for α-tubulin acetylation and for two distinct types of touch sensation. Furthermore, in animals lacking MEC-17, αTAT-2, and the sole C. elegans K40α-tubulin MEC-12, touch sensation can be restored by expression of an acetyl-mimic MEC-12[K40Q]. We conclude that αTAT1 is the major and possibly the sole α-tubulin K40 acetyltransferase in mammals and nematodes, and that tubulin acetylation plays a conserved role in several microtubule-based processes.

Effects of tubulin acetylation and tubulin acetyltransferase binding on microtubule structure.

Tubulin undergoes posttranslational modifications proposed to specify microtubule subpopulations for particular functions. Most of these modifications occur on the C-termini of tubulin and may directly affect the binding of microtubule-associated proteins (MAPs) or motors. Acetylation of Lys-40 on α-tubulin is unique in that it is located on the luminal surface of microtubules, away from the interaction sites of most MAPs and motors. We investigate whether acetylation alters the architecture of microtubules or the conformation of tubulin, using cryo-electron microscopy (cryo-EM). No significant changes are observed based on protofilament distributions or microtubule helical lattice parameters. Furthermore, no clear differences in tubulin structure are detected between cryo-EM reconstructions of maximally deacetylated or acetylated microtubules. Our results indicate that the effect of acetylation must be highly localized and affect interaction with proteins that bind directly to the lumen of the microtubule. We also investigate the interaction of the tubulin acetyltransferase, αTAT1, with microtubules and find that αTAT1 is able to interact with the outside of the microtubule, at least partly through the tubulin C-termini. Binding to the outside surface of the microtubule could facilitate access of αTAT1 to its luminal site of action if microtubules undergo lateral opening between protofilaments.

The oral-facial-digital syndrome gene C2CD3 encodes a positive regulator of centriole elongation.

Centrioles are microtubule-based, barrel-shaped structures that initiate the assembly of centrosomes and cilia. How centriole length is precisely set remains elusive. The microcephaly protein CPAP (also known as MCPH6) promotes procentriole growth, whereas the oral-facial-digital (OFD) syndrome protein OFD1 represses centriole elongation. Here we uncover a new subtype of OFD with severe microcephaly and cerebral malformations and identify distinct mutations in two affected families in the evolutionarily conserved C2CD3 gene. Concordant with the clinical overlap, C2CD3 colocalizes with OFD1 at the distal end of centrioles, and C2CD3 physically associates with OFD1. However, whereas OFD1 deletion leads to centriole hyperelongation, loss of C2CD3 results in short centrioles without subdistal and distal appendages. Because C2CD3 overexpression triggers centriole hyperelongation and OFD1 antagonizes this activity, we propose that C2CD3 directly promotes centriole elongation and that OFD1 acts as a negative regulator of C2CD3. Our results identify regulation of centriole length as an emerging pathogenic mechanism in ciliopathies.