Mechanisms of self-assembly and fibrillization of the prion-like domains

The mechanism of the self-assembly and fibrillization of the prion-like domains lies at the heart of their physiology and pathology. Here with the same methods previously established, we aimed to further decipher the mechanism by characterizing two prion-like sequences with the electrostatic properties very different from that of the full-length TDP-43 prion-like domain with a very basic pI value: namely the C-half of the TDP-43 prion-like domain only abundant in Gly, Ser, Asn and Gln with a pI of ~6.3, and the FUS prion-like domain enriched with Gly, Ser, Gln and Tyr with a pI of ~3.5. Interestingly, the C-half with the TDP-43 unique hydrophobic region removed is no longer able to form insoluble aggregates/fibrils but still capable of self-assembling into the reversible hydrogel with cross-{beta} structures, despite being much slower than the full-length. On the other hand, the FUS prion-like domain rapidly self-assembles into the reversible hydrogel with cross-{beta} fibrillar structures in 1 mM phosphate buffer at pH 6.8 but its self-assembly becomes very slow in 50 mM MES buffer at pH 5.5. Our study reveals that despite having completely different electrostatic properties, the full-length and C-half of the TDP-43 prion-like domain, as well as FUS prion-like domain all have the similar pH-dependence in self-assembly as we previously reported (Lim et al., [2016] PLOS Biol 14:e1002338). This unambiguously indicates that the self-assembly of the prion-like domains is not generally governed by the electrostatic interaction. Rather, their self-assembly and fibrillization are specified by the sequences despite being highly polar and degenerative. Furthermore, our study provides the first evidence that the formation of reversible hydrogel with cross-{beta} structures is separable from fibrillization of the prion-like domain. Finally, our results also successfully reconcile the previous discrepancy about the conformation and mechanism of the self-assembly of the FUS prion-like domain.