Crystallization and preliminary X-ray diffraction analysis of a single variable domain of the immunoglobulin superfamily in amphioxus, Amphi-IgSF-V.

Amphioxus is regarded as an essential animal model for the study of immune evolution. Discovery of new molecules with the immunoglobulin superfamily (IgSF) variable (V) domain in amphioxus would help in studying the evolution of IgSF V molecules in the immune system. A protein was found which just contains only one IgSF V domain in amphioxus, termed Amphi-IgSF-V; it has over 30% sequence identity to the V domains of human immunoglobulins and mammalian T-cell receptors. In order to clarify the three-dimensional structure of this new molecule in amphioxus, Amphi-IgSF-V was expressed, purified and crystallized, and diffraction data were collected to a resolution of 1.95 Å. The crystal belonged to space group P3221, with unit-cell parameters a = b = 53.9, c = 135.5 Å. The Matthews coefficient and solvent content were calculated to be 2.58 Å(3) Da(-1) and 52.38%, respectively. The results will provide structural information to study the evolution of IgSF V molecules in the immune system.