The cilium is a complex organelle, the assembly of which requires the coordination of motor-driven intraflagellar transport (IFT), membrane trafficking and selective import of cilium-specific proteins through a barrier at the ciliary transition zone. Recent findings provide insights into how cilia assemble and disassemble in synchrony with the cell cycle and how the balance of ciliary assembly and disassembly determines the steady-state ciliary length, with the inherent length-dependence of IFT rendering the ciliary assembly rate a decreasing function of length. As cilia are important in sensing and processing developmental signals and directing the flow of fluids such as mucus, defects in ciliogenesis and length control are likely to underlie a range of cilium-related human diseases.

Ciliogenesis: building the cell's antenna

Cells generate distinct microtubule subtypes by expressing different tubulin isotypes and through tubulin post-translational modifications, such as detyrosination, acetylation, polyglutamylation and polyglycylation The recent discovery of enzymes responsible for many of these modifications has shown how they may regulate microtubule functions

Post-translational regulation of the microtubule cytoskeleton: mechanisms and functions

S phase kinase-associated protein 1 (SKP1)-cullin 1 (CUL1)-F-box protein (SCF) ubiquitin ligase complexes use a family of F-box proteins as substrate adaptors to mediate the degradation of a large number of regulatory proteins involved in diverse processes The dysregulation of SCF complexes and their substrates contributes to multiple pathologies In the 14 years since the identification and annotation of the F-box protein family, the continued identification and characterization of novel substrates has greatly expanded our knowledge of the regulation of substrate targeting and the roles of F-box proteins in biological processes Here, we focus on the evolution of our understanding of substrate recruitment by F-box proteins, the dysregulation of substrate recruitment in disease and potential avenues for F-box protein-directed disease therapies

/pdf/mechanisms-and-function-of-substrate-recruitment-by-f-box-mvbsjr3p5q.pdf

Mechanisms and function of substrate recruitment by F-box proteins

Eukaryotic protein degradation by the proteasome and the lysosome is a dynamic and complex process in which ubiquitin has a key regulatory role. The distinctive morphology of the postmitotic neuron creates unique challenges for protein degradation systems with respect to cell-surface protein turnover and substrate delivery to proteolytic machineries that are required for both synaptic plasticity and self-renewal. Moreover, the discovery of ubiquitin-positive protein aggregates in a wide spectrum of neurodegenerative diseases underlines the importance and vulnerability of the degradative system in neurons. In this article, we discuss the molecular mechanism of protein degradation in the neuron with respect to both its function and its dysfunction.

Ubiquitin, the proteasome and protein degradation in neuronal function and dysfunction

Microtubules are cytoskeletal filaments that are dynamically assembled from α/β-tubulin heterodimers. The primary sequence and structure of the tubulin proteins and, consequently, the properties and architecture of microtubules are highly conserved in eukaryotes. Despite this conservation, tubulin is subject to heterogeneity that is generated in two ways: by the expression of different tubulin isotypes and by posttranslational modifications (PTMs). Identifying the mechanisms that generate and control tubulin heterogeneity and how this heterogeneity affects microtubule function are long-standing goals in the field. Recent work on tubulin PTMs has shed light on how these modifications could contribute to a “tubulin code” that coordinates the complex functions of microtubules in cells.

https://rupress.org/jcb/article-pdf/206/4/461/1365515/jcb_201406055.pdf

The tubulin code: molecular components, readout mechanisms, and functions.

https://www.cell.com/cell/pdf/S0092-8674(07)00902-6.pdf

SCRAPPER-dependent ubiquitination of active zone protein RIM1 regulates synaptic vesicle release

Airway epithelial cilia protect the mammalian respiratory system from harmful inhaled materials by providing the force necessary for effective mucociliary clearance. Ciliary beating is asymmetric, composed of clearly distinguished effective and recovery strokes. Neither the importance of nor the essential components responsible for the beating asymmetry has been directly elucidated. We report here that the beating asymmetry is crucial for ciliary function and requires tubulin glutamylation, a unique posttranslational modification that is highly abundant in cilia. WT murine tracheal cilia have an axoneme-intrinsic structural curvature that points in the direction of effective strokes. The axonemal curvature was lost in tracheal cilia from mice with knockout of a tubulin glutamylation-performing enzyme, tubulin tyrosine ligase-like protein 1. Along with the loss of axonemal curvature, the axonemes and tracheal epithelial cilia from these knockout (KO) mice lost beating asymmetry. The loss of beating asymmetry resulted in a reduction of cilia-generated fluid flow in trachea from the KO mice. The KO mice displayed a significant accumulation of mucus in the nasal cavity, and also emitted frequent coughing- or sneezing-like noises. Thus, the beating asymmetry is important for airway ciliary function. Our find-ings provide evidence that tubulin glutamylation is essential for ciliary function through the regulation of beating asymmetry, and provides insight into the molecular basis underlying the beating asymmetry.

https://www.pnas.org/content/pnas/107/23/10490.full.pdf

Tubulin polyglutamylation is essential for airway ciliary function through the regulation of beating asymmetry

Some ubiquitin-like (UBL) domain-containing proteins are known to play roles in receptor trafficking. Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) undergo constitutive cycling between the intracellular compartment and the cell surface in the central nervous system. However, the function of UBL domain-containing proteins in the recycling of the AMPARs to the synaptic surface has not yet been reported.

Here, we report that the Transmembrane and ubiquitin-like domain-containing 1 (Tmub1) protein, formerly known as the Hepatocyte Odd Protein Shuttling (HOPS) protein, which is abundantly expressed in the brain and which exists in a synaptosomal membrane fraction, facilitates the recycling of the AMPAR subunit GluR2 to the cell surface. Neurons transfected with Tmub1/HOPS-RNAi plasmids showed a significant reduction in the AMPAR current as compared to their control neurons. Consistently, the synaptic surface expression of GluR2, but not of GluR1, was significantly decreased in the neurons transfected with the Tmub1/HOPS-RNAi and increased in the neurons overexpressing EGFP-Tmub1/HOPS. The altered surface expression of GluR2 was speculated to be due to the altered surface-recycling of the internalized GluR2 in our recycling assay. Eventually, we found that GluR2 and glutamate receptor interacting protein (GRIP) were coimmunoprecipitated by the anti-Tmub1/HOPS antibody from the mouse brain.

Taken together, these observations show that the Tmub1/HOPS plays a role in regulating basal synaptic transmission; it contributes to maintain the synaptic surface number of the GluR2-containing AMPARs by facilitating the recycling of GluR2 to the plasma membrane.

/pdf/transmembrane-and-ubiquitin-like-domain-containing-protein-1-1gm5hlheia.pdf

Showbu Sato

Papers

SCRAPPER-dependent ubiquitination of active zone protein RIM1 regulates synaptic vesicle release

Tubulin polyglutamylation is essential for airway ciliary function through the regulation of beating asymmetry

Transmembrane and Ubiquitin-Like Domain-Containing Protein 1 (Tmub1/HOPS) Facilitates Surface Expression of GluR2-Containing AMPA Receptors

SCRAPPER-Dependent Ubiquitination of Active Zone Protein RIM1 Regulates Synaptic Vesicle Release (DOI:10.1016/j.cell.2007.06.052)