Beta Amyloid ~The truncated C-terminal RRM domain of TDP-43 plays a key role in forming proteinaceous aggregates.

TDP-43 is the major pathological protein identified in the cellular inclusions in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). The pathogenic forms of TDP-43 are processed C-terminal fragments containing a truncated RNA-recognition motif (RRM2) and a glycine-rich region. Although extensive studies have focused on this protein, it remains unclear how the dimeric full-length TDP-43 is folded and assembled and how the processed C-terminal fragments are misfolded and aggregated. Here using size-exclusion chromatography, pull down assays and small angle X-ray scattering, we show that the C-terminal deleted TDP-43 without the glycine-rich tail is sufficient to form a head-to-head homodimer primarily via its N-terminal domain. The truncated RRM2, as well as two β-strands within the RRM2, form fibrils in vitro with a similar amyloid-negative staining property to those of TDP-43 pathogenic fibrils in diseases. In addition to the glycine-rich region, the truncated RRM2, but not the intact RRM2, plays a key role in forming cytoplasmic inclusions in neuronal cells. Our data thus suggest that the process that disrupts the dimeric structure, such as the proteolytic cleavage of TDP-43 within the RRM2 that removes the N-terminal dimerization domain, may produce unassembled truncated RRM2 fragments with abnormally exposed β-strands which can oligomerize into high-order inclusions.