The membrane topology problem inherent in ciliogenesis is solved by ciliary recruitment of a phospholipid flippase

The cilium is a slender cellular projection whose membrane transduces signals essential in development and homeostasis. Although assembly of the axonemal core of the organelle is well-studied, the mechanisms that underlie formation of the ~250 nm diameter ciliary membrane from oppositely curved 50-100 nm diameter pre-ciliary vesicles remain poorly understood. Here we report that during ciliogenesis in the bi-ciliated alga, Chlamydomonas reinhardtii, aminophospholipid flippase 2 (ALA2) is recruited laterally from a reservoir on the plasma membrane onto the nascent ciliary membrane where it transfers conically shaped phosphatidyl ethanolamine (PE) from the outer to the inner leaflet. In cells lacking ALA2, initiation of cilia regeneration after experimentally induced de-ciliation is delayed, PE flipping is impaired, the ciliary membrane becomes distorted, and cilia that do form are shorter than wild type. Our results uncover a membrane topology problem inherent in ciliogenesis and demonstrate that it is resolved by a phospholipid flippase.