Expression, purification and structural characterization of three human proteins: apolipoprotein M, heme-binding protein 2 and folate receptor α

This thesis work was aimed at the expression, purification and crystallization of three human proteins (apolipoprotein M, folate receptor α and SOUL protein) in order to determine their three-dimensional structure by means of X-ray protein crystallography. Human apolipoprotein M was expressed using the methylotrophic yeast P. pastoris. The recombinant protein was purified by ion-exchange chromatography, preparative isoelectric focusing, gel filtration, and Lipidex-1000 chromatography. In order to obtain a more homogeneous protein, the non-glycosylated mutant (Asn135Gln) was also expressed and purified. The crystallization trials gave some positive results with mutated apoM, although the crystals are still not suitable for X-ray diffraction experiments. The heterologous expression of the human FR-a was troublesome, and although different expression systems (P. pastoris, baculovirus, and N. benthamiana) were tested, only a low amount of recombinant protein was obtained (from P. pastoris) and purified (by ion-exchange chromatography and gel filtration). However non of the crystallization conditions tested was successful, probably due to the low protein concentration. Human SOUL (heme-binding protein 2) was expressed in E. coli and purified by immobilized metal ion affinity chromatography, using the hexa-histidine tag added to the C-terminus of the protein. The recombinant SOUL was crystallized both as apoprotein and as a complex in the presence of hemin. The preliminary X-ray diffraction analysis shows the presence of six molecules in the unit cell, and no significant differences between the apoand the holoprotein were found. Further studies suggest that hemin is not bound to the protein, since the Fe peak could not be found in the X-ray fluorescence spectrum of the crystals. Attempts to solve the three-dimensional structure by means of multiple isomorphous replacement, multiwavelength anomalous diffraction and molecular replacement are still in progress.