Open Sesame: How Transition Fibers and the Transition Zone Control Ciliary Composition

Abstract

<p class="first" id="d1911890e109">Cilia are plasma membrane protrusions that act as cellular propellers or antennae. To perform these functions, cilia must maintain a composition distinct from those of the contiguous cytosol and plasma membrane. The specialized composition of the cilium depends on the ciliary gate, the region at the ciliary base separating the cilium from the rest of the cell. The ciliary gate's main structural features are electron dense struts connecting microtubules to the adjacent membrane. These structures include the transition fibers, which connect the distal basal body to the base of the ciliary membrane, and the Y-links, which connect the proximal axoneme and ciliary membrane within the transition zone. Both transition fibers and Y-links form early during ciliogenesis and play key roles in ciliary assembly and trafficking. Accordingly, many human ciliopathies are caused by mutations that perturb ciliary gate function. </p><p class="first" id="d1911890e112">The ciliary gate is the region at the base of the cilium that separates it from the rest of the cell. The gate consists of transition fibers and a transition zone that regulate proteins entering and exiting the cilium. </p>