Proteomic Analysis of Isolated Ciliary Transition Zones Reveals the Presence of ESCRT Proteins

The transition zone (TZ) is a specialized region of the cilium characterized by Y-shaped connectors between the microtubules of the ciliary axoneme and the ciliary membrane [1]. Located near the base of the cilium, the TZ is in the prime location to act as a gate for proteins into and out of the ciliary compartment, a role supported by experimental evidence [2–6]. The importance of the TZ has been underscored by studies showing that mutations affecting proteins located in the TZ result in cilia-related diseases, or ciliopathies, presenting symptoms including renal cysts, retinal degeneration, and situs inversus [7–9]. Some TZ proteins have been identified and shown to interact with each other through coprecipitation studies in vertebrate cells [4, 10, 11] and genetics studies in C. elegans [3]. As a distinct approach to identify TZ proteins, we have taken advantage of the biology of Chlamydomonas to isolate TZs. Proteomic analysis identified 115 proteins, ten of which were known TZ proteins related to ciliopathies, indicating that the preparation was highly enriched for TZs. Interestingly, six proteins of the endosomal sorting complexes required for transport (ESCRT) were also associated with the TZs. Identification of these and other proteins in the TZ will provide new insights into functions of the TZ, as well as candidate ciliopathy genes. •Ciliary transition zones were isolated from Chlamydomonas•Proteomic analysis of the transition zones identified 115 proteins•Six ESCRT proteins were identified associated with the transition zone Defects in the transition zone, a specialized region found at the base of cilia, result in cilia-related diseases. Diener et al. report a proteomic analysis of isolated transition zones that revealed the presence of ESCRT proteins, suggesting that these proteins play a previously unrecognized role in ciliary membrane dynamics.