Abstract
The primary cilium is a solitary, microtubule-based membrane protrusion extending from the surface of quiescent cells that senses the cellular environment and triggers specific cellular responses. The functions of primary cilia require not only numerous different components but also their regulated interplay. The cilium performs highly dynamic processes, such as cell cycle-dependent assembly and disassembly as well as delivery, modification, and removal of signaling components to perceive and process external signals. On a molecular level, these processes often rely on a stringent control of key modulatory proteins, of which the activity, localization, and stability are regulated by post-translational modifications (PTMs). While an increasing number of PTMs on ciliary components are being revealed, our knowledge on the identity of the modifying enzymes and their modulation is still limited. Here, we highlight recent findings on cilia-specific phosphorylation and ubiquitylation events. Shedding new light onto the molecular mechanisms that regulate the sensitive equilibrium required to maintain and remodel primary cilia functions, we discuss their implications for cilia biogenesis, protein trafficking, and cilia signaling processes.
Highlights
Primary cilia are dynamic cellular signaling compartments of the plasma membrane (Garcia et al, 2018; Anvarian et al, 2019) composed of a membrane-surrounded microtubule core, termed the axoneme
We focus on phosphorylation and ubiquitylation and discuss recent findings on their involvement in regulating cilia formation and signaling
One hallmark ciliary signaling pathway that highlights the dynamics in post-translational modifications (PTMs) is Hedgehog signaling in vertebrates (Figure 1; Gigante and Caspary, 2020)
Summary
Primary cilia are dynamic cellular signaling compartments of the plasma membrane (Garcia et al, 2018; Anvarian et al, 2019) composed of a membrane-surrounded microtubule core, termed the axoneme. Phosphorylation can modulate interaction surfaces or lead to intramolecular rearrangements that alter enzymatic activities. Protein kinases phosphorylate their substrates at specific consensus sites consisting of only a few amino acids. They are often targets of phosphorylation themselves, which results in phosphorylation cascades that are typically found in cellular signaling processes (Miller and Turk, 2018). We focus on phosphorylation and ubiquitylation and discuss recent findings on their involvement in regulating cilia formation and signaling
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