Effects of fusion tags on protein partitioning In aqueous two-phase systems and use in primary protein recovery

Sammanfattning: The two techniques aqueoustwo-phase partitioning and expanded bed adsorption that bothare suitable for primary protein recovery were studied. Most ofthe work was focused on partition in aqueous two-phase systemsand in particular on the possibility to effect the partitionbehaviour by fusion of short peptide tags or protein domains tothe target protein.The partitioning of fusionproteins between different variants of the domain tag Z and thenaturally occurring protein DNA Klenow polymerase were studiedin Breox/Reppal aqueous two-phase systems. Most studies wereperformed with cell homogenate. The Breox/Reppal system was infocus because if the fusion protein can be partitioned to theBreox-rich top phase the next step can be a thermoseparatingaqueous two-phase system. When the Breox phase is heated to50°C it switches from a one-phase system to a two-phasesystem resulting in an almost pure water rich top phase andhighly concentrated Breox-rich bottom phase. The Breox can thenbe reused and the protein recovered from the water phase. TheZ-domain was genetically modified in different ways to Zbasic1, Zacid2and Ztrp12and fused to the Klenow protein to try toenhance partitioning to the Breox-rich phase. From theexperiments it was not possible to observe any effects on thepartition behaviour irrespectively of tested properties of thedomain tag. Despite the absence of domain tag effects highK-values, i.e. partition to the Breox-rich top phase, wereobserved in the Breox/Reppal system. However, the proteinK-values seemed to be rather sensitive to the cell homogenateload and showed a tendency to decrease with increased cellhomogenate load. Also increased phosphate concentration reducedthe K-values. The partitioning of cell debris also seemed todependent on the cell homogenate load. At higher homogenateload (<=20g DW/L) clear Breox-rich top phases were observedwith the cell debris collected in Reppal-rich bottomphases.Two different tetrapeptides,AlaTrpTrpPro and AlaIleIlePro were inserted near the C-terminusof the protein ZZT0. The Trp-rich peptide unit stronglyincreased both the partitioning of ZZT0 into the poly(ethyleneglycol) (PEG)-rich phase in a PEG/potassium phosphate aqueoustwo-phase system and its retention on PEG and propylhydrophobic interaction chromatographic columns with potassiumphosphate as eluent in isocratic systems. Both the partitioningand the retention increased with increasing number of Trp-richpeptide units inserted into ZZT0. Insertion of Ile-richtetrapeptide units affected the partitioning and retention to amuch lesser extent. Partition and modelling data also indicateda folding of inserted Trp and Ile tetrapeptide units, probablyto minimise their water contact. It was also investigated howto predict the partitioning of proteins in isoelectricPEG/phosphate aqueous two-phase systems.The capture ofß-galactosidase fromE. colicell homogentate (50g DW/L) by metal chelatexpanded bed adsorption was studied. These experiments showedthat capture, with a certain degree of selectivity, andclarification of ß-galactosidase could be achieved from acell homogenate. However, a rather low recovery of about 35 %was obtained at a capacity of 0.25mg/mL of gel. Thus, severalparameters remain to be optimised like the load buffercomposition and the cell homogenate load.Keywords:E. coli, aqueous two-phase systems, fusion proteins,hydrophobic interaction chromatography, expanded bedadsorption, ß-galactosidase, Klenow polymerase, Z-domain,peptide tags