TRPC4 and TRPC5 are two closely related members of the mammalian transient receptor potential cation channel family that have been implicated in important physiological functions, such as growth cone guidance and smooth muscle contraction. To further unravel the role of TRPC4 and TRPC5 in these processes in vivo, detailed information about the molecular composition of native channel complexes and their association with cellular signaling networks is needed. We therefore searched a human aortic cDNA library for novel TRPC4-interacting proteins using a modified yeast two-hybrid assay. This screen identified SESTD1, a previously uncharacterized protein containing a lipid-binding SEC14-like domain as well as spectrin-type cytoskeleton interaction domains. SESTD1 was found to associate with TRPC4 and TRPC5 via the channel's calmodulin- and inositol 1,4,5-trisphosphate receptor-binding domain. In functional studies, we demonstrate that SESTD1 binds several phospholipid species in vitro and is essential for efficient receptor-mediated activation of TRPC5. Notably, phospholipid binding to SESTD1 was Ca(2+)-dependent. Because TRPC4 and -5 conduct Ca(2+), SESTD1-channel signaling may be bidirectional and also couple TRPC activity to lipid signaling through SESTD1. The modulation of TRPC channel function by specific lipid-binding proteins, such as SESTD1, adds another facet to the complex regulation of these channels complementary to the previously described effects of direct channel-phospholipid interaction.