Primary cilium transition zone

The primary cilium transition zone lies between basal body and the primary cilium. Its sieve-like nature limits free diffusion of proteins into and out of the primary cilium. Other components of the transition zone may also limit the diffusion of molecules, and lipids on the plasma membrane. Ultrastructurally, three major components of the transition zone have been described that contribute to its barrier function: (1) At the transition zone, the ciliary membrane is connected to the microtubules of the axoneme by Y-shaped protein complexes (so-called Y-links), (2) below the transition zone, the distal appendage matrix seals the space between transition fibers, which mount the basal body to the plasma membrane, and (3) a septin-based barrier between the ciliary membrane and the plasma membrane limits exchange of membrane proteins. Little is known about the exclusion size and the molecular nature of the transition zone barrier. Only smaller proteins (molecular mass 100 kDa) and calcium ions have been observed to diffuse into and out of the cilium.

Immunofluorescent staining

The transition zone is such a tiny structure that its components can barely be resolved with fluorescence microsocpy. In ciliated cells, the transition zone can be seen as a fine line or spot between the basal body and the primary cilium.

Read more about the proteome of the primary cilium transition zone as a substructure of the primary cilium.