The ciliary membrane

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Cilia and flagella are important organizing centers for signaling in both development and disease. A key to their function is a poorly characterized barrier at their base that allows the protein and lipid composition of the ciliary membrane to be distinct from that of the plasma membrane. We review current models of ciliary membrane biogenesis, highlighting several structures, including the ciliary necklace and ciliary pocket, that appear during biogenesis and that likely contribute to the barrier. The regulated movement of membrane proteins and lipids across this barrier is central to the sensory function of these organelles.

Section snippets

The cilium is a signaling compartment for the receipt of extracellular signals

In many ways, we experience our environment through cilia. The outer segments of photoreceptor cells, whose responsiveness to light approaches single photon sensitivity, are modified cilia; the kinocilium in hair cells of the ear organizes the stereociliary bundles that detect sound waves; and odorant reception occurs on the cilia in the olfactory epithelium. Cilia are also involved in the detection of signals produced within the organism, suggested by the enrichment of various receptors in the

The ciliary necklace and the ciliary pocket are sites of intimate membrane–basal body interactions

Membrane proteins and lipids that enter the cilium must traverse two distinct membrane specializations near the base of the cilium that are sites of membrane–basal body interactions. These regions constitute the functional barrier that separates the ciliary membrane from the plasma membrane. Both regions are near the transition zone, the site in the basal body at which the triplet microtubules of the basal body transition to the doublet microtubules of the axoneme. The most distal is the

The ciliary pocket and its associated structures likely regulate membrane protein and lipid entry into the primary cilium

The ciliary pocket, necklace, and transitional fibers likely each make important contributions to the barrier that prevents the free mixing of membrane proteins between the plasma membrane and the ciliary membrane. The highly curved nature of the membrane at the base of the pocket could itself impose geometric constraints on the movement of lipids and membrane proteins across this region. The apparently stable, intimate interaction between the transitional fibers and the membrane could also

Transport pathways for membrane protein movement to primary cilia

Current models suggest that membrane proteins targeted to cilia are deposited by vesicles near the base of the cilium (reviewed in [30]). The most likely place for vesicle fusion in this case is the membrane of the ciliary pocket itself (Figure 1d); however, this has not been rigorously established for primary cilia. Early insight into this process came from the study of ciliary appendages called mastigonemes from certain flagellates [31]. Using EM, these structures were found to be transported

Concluding remarks

Many of our current ideas about the ciliary membrane rest on studies performed decades ago. An exciting frontier in ciliary biology is the integration of this largely ultrastructural information with the recent explosion in the discovery of molecular components required for cilia formation and function.

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

We express our thanks to Olivier Belzile in the Snell Laboratory for providing the original electron micrographs in Figure 1. Figure 1A is an adaptation from Gilula and Satir [10]. R. Rohatgi is supported in part by NIH ROO CA129174 and WJ Snell is supported in part by NIH GM 25661.

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