Structure of mistletoe lectin I from Viscum album in complex with the phytohormone zeatin

The crystal structure of mistletoe lectin I (ML-I) isolated from the European mistletoe Viscum album in complex with the most active phytohormone zeatin has been analyzed and refined to 2.54 Å resolution. X-ray suitable crystals of ML-I were obtained by the counter-diffusion method using the Gel-Tube R crystallization kit (GT-R) onboard the Russian Service Module on the international space station ISS. High quality hexagonal bipyramidal crystals were grown during 3 months under microgravity conditions. Selected crystals were soaked in a saturated solution of zeatin and subsequently diffraction data were collected applying synchrotron radiation. A distinct Fo-Fc electron density has been found inside a binding pocket located in subunit B of ML-I and has been interpreted as a single zeatin molecule. The structure was refined to investigate the zeatin–ML-I interactions in detail. The results demonstrate the ability of mistletoe to protect itself from the host transpiration regulation by absorbing the most active host plant hormones as part of a defense mechanism.