Profiling truncated variants of TCTN1 unveils the essential role of its integrity for ciliogenesis

Cilia protrude from the cell surface to execute signal transduction and cell motility. Tectonic (TCTN) is a transition-zone (TZ) "gatekeeper" that determines ciliary composition and function. The role of its only conserved module in the TCTN protein family, DUF1619, lacks functional annotation. Herein, we quantitatively compared full-length TCTN1 and four truncations (DUF1619, ΔDUF1619, N-DUF1619, and DUF1619-C) stably expressed in <i>Chlamydomonas reinhardtii tctn1</i> cells. Surprisingly, neither fragment could rescue ciliary morphology of <i>tctn1</i>; DUF1619 alone aggravates ciliary defects. In particular, deletion of either terminus decreases TZ localization and, for the C-terminus, destabilizes the protein during ciliary disassembly. Thus, our findings demonstrate that the full-length TCTN1, rather than DUF1619 alone, is indispensable for ciliary integrity.