| Summary: | The ΔN6 truncation is the main posttranslational modification of β<sub>2</sub>-microglobulin (β<sub>2</sub>M) found in dialysis-related amyloid. Investigation of the interaction of wild-type (WT) β<sub>2</sub>M with N-terminally truncated variants is therefore of medical relevance. However, it is unclear which residues among the six residues at the N-terminus are crucial to the interactions and the modulation of amyloid fibril propagation of β<sub>2</sub>M. We herein analyzed homo- and heterotypic seeding of amyloid fibrils of WT human β<sub>2</sub>M and its N-terminally-truncated variants ΔN1 to ΔN6, lacking up to six residues at the N-terminus. At acidic pH 2.5, we produced amyloid fibrils in vitro from recombinant, WT β<sub>2</sub>M and its six truncated variants, and found that ΔN6 β<sub>2</sub>M fibrils exhibit a significantly lower conformational stability than WT β<sub>2</sub>M fibrils. Importantly, under more physiological conditions (pH 6.2), we assembled amyloid fibrils in vitro only from recombinant, ΔN4, ΔN5, and ΔN6 β<sub>2</sub>M but not from WT β<sub>2</sub>M and its three truncated variants ΔN1 to ΔN3. Notably, the removal of the six, five or four residues at the N-terminus leads to enhanced fibril formation, and homo- and heterotypic seeding of ΔN6 fibrils strongly promotes amyloid fibril formation of WT β<sub>2</sub>M and its six truncated variants, including at more physiological pH 6.2. Collectively, these results demonstrated that the residues 4 to 6 at the N-terminus particularly modulate amyloid fibril propagation of β<sub>2</sub>M and the interactions of WT β<sub>2</sub>M with N-terminally truncated variants, potentially indicating the direct relevance to the involvement of the protein’s aggregation in dialysis-related amyloidosis.
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