Fractions were tested for purity by SDS-PAGE and the cleanest samples were pooled and dialyzed against MSP buffer (20 mM Tris-HCl, 100 mM NaCl, 0.5 mM EDTA, pH 7.4) at 4 °C. Finally, the sample was filtered using a 0.22 μm membrane, aliquoted, flash frozen with liquid nitrogen and stored at -80 °C.