Overexpressed TTC3 Protein Tends to be Cleaved into Fragments and Form Aggregates in the Nucleus

Human tetratricopeptide repeat domain 3 (TTC3) is a gene on 21q22.2 within the Down syndrome critical region (DSCR). Earlier studies suggest that TTC3 may be an important regulator in individual development, especially in neural development. As an E3 ligase, TTC3 binds to phosphorylated Akt and silence its activity via proteasomal cascade. Several groups also reported the involvement of TTC3 in familial Alzheimer's disease recently. In addition, our previous work shows that TTC3 also regulates the degradation of DNA polymerase gamma and over-expressed TTC3 protein tends to form insoluble aggregates in cells. In this study, we focus on the solubility and intracellular localization of TTC3 protein. Over-expressed TTC3 tends to form insoluble aggregates over time. The proteasome inhibitor MG132 treatment resulted in more TTC3 aggregates in a short period of time. We fused the fluorescent protein to either terminus of the TTC3 protein and found that the intracellular localization of fluorescent signals are different between the N-terminal tagged and C-terminal tagged proteins. Western blotting revealed that the TTC3 protein is cleaved into fragments of different sizes at multiple sites. The N-terminal sub-fragments of TTC3 are prone to from nuclear aggregates and the TTC3 nuclear import is mediated by signals within the N-terminal 1 to 650 residues. Moreover, over-expressed TTC3 induced a considerable degree of cytotoxicity, and its N-terminal sub-fragments are more potent inhibitors of cell proliferation than full-length protein. Considering the prevalent proteostasis dysregulation in neurodegenerative diseases, these findings may relate to the pathology of such diseases.