Abstract
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 A 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 H(2)O(2) (K(m,NAPDH) = 222 microM; k(cat) = 77 sec(-1)), a molecule with known signaling properties. We propose that the H(2)O(2) produced by this reaction may be one of the signaling molecules involved in axon guidance by MICAL.