This is a conserved domain found in tectonic proteins (TCTN1/2/3) which contains a Cys rich N-terminal region. Although its function is currently unknown (this domain is also known as DUF1619), studies in TCTN2 suggest that it is indispensable for normal functions [1,2]. These proteins form a complex required for hedgehog signalling transduction [3].

1: Super-Resolution Imaging Reveals TCTN2 Depletion-Induced IFT88 Lumen Leakage and Ciliary Weakening. Weng RR, Yang TT, Huang CE, Chang CW, Wang WJ, Liao JC; Biophys J. 2018;115:263-275. PMID:29866362

2: Open Sesame: How Transition Fibers and the Transition Zone Control Ciliary Composition. Garcia-Gonzalo FR, Reiter JF; Cold Spring Harb Perspect Biol. 2017; [Epub ahead of print] PMID:27770015

3: A ciliopathy complex at the transition zone protects the cilia as a privileged membrane domain. Chih B, Liu P, Chinn Y, Chalouni C, Komuves LG, Hass PE, Sandoval W, Peterson AS; Nat Cell Biol. 2011;14:61-72. PMID:22179047