The Hedgehog receptors PTCH1 and PTCH2 exist as active homomeric and heteromeric complexes

With the importance of Hedgehog signalling in embryonic development, tissue homeostasis and disease, understanding the molecular mechanisms of signal transduction is paramount for the design of specific, effective therapeutics. The Hedgehog receptor PTCH1 and the less studied PTCH2 isoform are evolutionarily related to bacterial RND permeases and sterol sensing proteins, which mobilise hydrophobic compounds powered by a cation gradient. Here we demonstrate that, in the active state, PTCH1 and PTCH2 form homomeric and heteromeric complexes that are inhibited by binding of Sonic Hedgehog. We show that PTCH2, unlike PTCH1, appears to have minimal cholesterol transport activity, but that conserved residues involved in cation transport are essential for its function. Heteromeric PTCH1-PTCH2 complexes depend on PTCH1s cholesterol transport capacity, but the cation transport can be provided in trans by PTCH2, suggesting that some deleterious mutations in either isoform can be silenced by formation of heteromers, enhancing the robustness of this signal transduction system. These findings provide the molecular basis for the intriguing behaviour of PTCH2 as semi-redundant and partially overlapping in function with PTCH1 and explain the dominant negative effect of mutations that disrupt the PTCH2 cation transport triad in rare cases of cancer.