The nuclear accumulation of proteins may depend on the presence of short targeting sequences, which are known as nuclear localization signals (NLSs). Here, we found that NLSs are predicted in some cytosolic proteins and examined the hypothesis that these NLSs may be functional under certain conditions. As a model, human cardiac troponin I (hcTnI) was used. After expression in cultured non-muscle or undifferentiated muscle cells, hcTnI accumulated inside nuclei. Several NLSs were predicted and confirmed by site-directed mutagenesis in hcTnI. Nuclear import occurred via the classical karyopherin-α/β nuclear import pathway. However, hcTnI expressed in cultured myoblasts redistributed from the nucleus to the cytoplasm, where it was integrated into forming myofibrils after the induction of muscle differentiation. It appears that the dynamic retention of proteins inside cytoplasmic structures can lead to switching between nuclear and cytoplasmic localization.

中文翻译：

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肌肉分化过程中心肌肌钙蛋白I在核和细胞质定位之间的转换。

蛋白质的核积累可能取决于短靶向序列的存在，这些短靶向序列被称为核定位信号（NLS）。在这里，我们发现某些胞质蛋白中预测了NLS，并检验了这些NLS在某些条件下可能起作用的假设。作为模型，使用了人类心脏肌钙蛋白I（hcTnI）。在培养的非肌肉或未分化的肌肉细胞中表达后，hcTnI积累在细胞核内。通过在hcTnI中进行定点诱变，可以预测并确认几种NLS。核输入通过经典的karyopherin-α/β核输入途径发生。但是，hcTnI在培养的成肌细胞中表达，从细胞核重新分布到细胞质，在诱导肌肉分化后，hcTnI被整合成肌原纤维。