Possible mechanisms of polyphosphate-induced amyloid fibril formation of β2-microglobulin

Polyphosphate (polyP), which is found in various microorganisms and human cells, is an anionic biopolymer consisting of inorganic phosphates linked by high-energy phosphatebonds. Previous studies revealed that polyPs strongly promoted the amyloidformation of several amyloidogenic proteins; however, the mechanism of polyP-induced amyloidformation remains unclear. In the present study using β 2 -microglobulin (β2m), a protein responsible for dialysis-related amyloidosis, we investigated amyloidformation in the presence of various chain lengths of polyPs at different concentrations under both acidic (pH 2.0 to 2.5) and neutral pH (pH 7.0 to 7.5) conditions. We found that the amyloidformation of β2m at acidic pH was significantly accelerated by the addition of polyPs at an optimal polyP concentration, which decreased with an increase in chain length. The results obtained indicated that electrostatic interactions between positively charged β2m and negatively charged polyPs play a major role in amyloidformation. Under neutral pH conditions, long polyP with 60 to 70 phosphates induced the amyloidformation of β2m at several micromoles per liter, a similar concentration range to that in vivo. Since β2m with an isoelectric point of 6.4 has a slightly negative net charge at pH 7, polyPs were unlikely to interact with β2m electrostatically. PolyPs appear to dehydrate water molecules around β2m under the unfolded conformation, leading to the preferential stabilization of less water-exposed amyloidfibrils. These results not only revealed the pH-dependent mechanism of the amyloidformation of β2m but also suggested that polyPs play an important role in the development of dialysis-related amyloidosis.