Properties of Some Variants of Human β2- Microglobulin and Amyloidogenesis

Three variants of human β2-Microglobulin (β 2-m) were compared with wild-type protein. For two variants, namely the mutant RSAβ2-m and the form devoid of the N-terminal tripeptide (ΔN3β2-m), a reduced unfolding free energy was measured compared with wild-type βm, whereas an increased stability was observed for the mutant H31Yβ2-m. The solution structure could be determined by 1H NMR spectroscopy and restrained modeling only for RSAβ2m that showed the same conformation as the parent species, except for deviations at the interstrand loops. Analogous conclusions were reached for H31Yβ2-m and ΔN3β 2-m. Precipitation and unfolding were observed over time periods shorter than 4-6 weeks with all the variants and, sometimes, with wild-type protein. The rate of structured protein loss from solution as a result of precipitation and unfolding always showed pseudo-zeroth order kinetics. This and the failure to observe an unfolded species without precipitation suggest that a nucleated conformational conversion scheme should apply for β 2-m fibrillogenesis. The mechanism is consistent with the previous and present results on β2-m amyloid transition, provided a nucleated oligomeric species be considered the stable intermediate of fibrillogenesis, the monomeric intermediate being the necessary transition step along the pathway from the native protein to the nucleated oligomer.