Structure and activity of the axon guidance protein MICAL

During development, neurons are guided to their targets by short- and long-range attractive and repulsive cues. MICAL, a large multidomain protein, is required for the combined action of semaphorins and plexins in axon guidance. Here, we present the structure of the N-terminal region of MICAL (MICAL _fd) determined by x-ray diffraction to 2.0 Å resolution. The structure shows that MICAL_fd is an FAD-containing module structurally similar to aromatic hydroxylases and amine oxidases. In addition, we present biochemical data that show that MICAL_fd is a flavoenzyme that in the presence of NADPH reduces molecular oxygen to H₂O₂ (K _m,NAPDH = 222 μM; k_cat = 77 sec^-1), a molecule with known signaling properties. We propose that the H ₂O₂ produced by this reaction may be one of the signaling molecules involved in axon guidance by MICAL.