The immunoglobulin superfamily protein neurolin plays a central role during differentiation and development of retina ganglion cells in goldfish. As shown in earlier work, blockage of the second immunoglobulin domain (Ig2) of neurolin with domain-specific antibodies causes severe pathfinding defects of growing axons in the retina. Thus Ig2 of neurolin was identified as the critical domain for axon guidance. In the present study we have developed a protocol for expression and purification of neurolin-Ig2 suitable for structure analysis, functional studies and ligand identification. Neurolin was expressed in Rosettagami and Origaini strains of Escherichia coli which is deficient in glutathione- and thioredoxin reductase facilitating proper formation of the disulfide bond in the cytoplasm. The protein was purified via an N-terminal His(6)-tag by Ni2+ affinity and size exclusion chromatography. After purification the His(6)-tag was cut-off without loss of solubility. Analytical size exclusion chromatography revealed an apparent molecular mass for neurolin-Ig2 in agreement with a non-covalent homodimer. Analysis of CD and FTIR spectra gave a secondary structure content typical for Ig domains. (C) 2008 Elsevier Inc. All rights reserved.