Insertional mutations leading to expansion of the octarepeat domain of the
prion protein (PrP) are directly linked to
prion disease. While normal PrP has four PHGGGWGQ octapeptide segments in its flexible N-terminal domain, expanded forms may have up to nine additional octapeptide inserts. The type of
prion disease segregates with the degree of expansion. With up to four extra octarepeats, the average onset age is above 60 years, whereas five to nine extra octarepeats results in an average onset age between 30 and 40 years, a difference of almost three decades. In wild-type PrP, the octarepeat domain takes up
copper (Cu(2+)) and is considered essential for in vivo function. Work from our lab demonstrates that the
copper coordination mode depends on the precise ratio of Cu(2+) to
protein. At low Cu(2+) levels, coordination involves
histidine side chains from adjacent octarepeats, whereas at high levels each repeat takes up a single
copper ion through interactions with the
histidine side chain and neighboring backbone
amides. Here we use both octarepeat constructs and recombinant PrP to examine how
copper coordination modes are influenced by octarepeat expansion. We find that there is little change in affinity or coordination mode populations for octarepeat domains with up to seven segments (three inserts). However, domains with eight or nine total repeats (four or five inserts) become energetically arrested in the multi-
histidine coordination mode, as dictated by higher
copper uptake capacity and also by increased binding affinity. We next pooled all published cases of human
prion disease resulting from octarepeat expansion and find remarkable agreement between the sudden length-dependent change in
copper coordination and onset age. Together, these findings suggest that either loss of PrP
copper-dependent function or loss of
copper-mediated protection against PrP polymerization makes a significant contribution to early onset
prion disease.