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 β_m, 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.