Essential sequence of the N-terminal cytoplasmic localization-related domain of huntingtin and its effect on huntingtin aggregates.

Huntington's disease (HD) is caused by abnormal CAG repeat expansion in the 5'-end of the Huntingtin (HTT) gene. In addition to the canonical C-terminal full-length huntingtin (htt) nuclear export signal, a cytoplasmic localization-related domain (CLRD) in the N-terminus of htt has recently been reported. Here, we analyzed this domain by introducing deletion and substitution mutations in a truncated N-terminal htt protein and subsequently monitored htt expression, aggregation and subcellular localization by immunocytochemistry and Western blot analysis. We demonstrated that Htt(4-17) was the essential sequence for htt cytoplasmic localization. We also found that the subcellular distribution of htt was altered when Htt(1-17) was mutated to contain amino acids of different charges, suggesting a structural requirement of Htt(1-17) for the cytoplasmic localization of htt. Deletion of the first three amino acids did not affect its association with mitochondria. We observed that defective cytoplasmic localization resulted in a reduction of total htt aggregates and increased nuclear aggregates, indicating that the subcellular distribution of the protein might influence the aggregation process. These studies provide new insight into the molecular mechanism of htt aggregation in HD.