MLH1 and PMS2 proteins form the MutLα heterodimer, which plays a major role in DNA mismatch repair (MMR) in humans. Mutations in MMR-related proteins are associated with cancer, especially with colon cancer. The N-terminal region of MutLα comprises the N-termini of PMS2 and MLH1 and, similarly, the C-terminal region of MutLα is composed by the C-termini of PMS2 and MLH1, and the two are connected by linker region. The nuclear localization sequences (NLSs) necessary for the nuclear transport of the two proteins are found in this linker region. However, the exact NLS sequences have been controversial, with different sequences reported, particularly for MLH1. The individual components are not imported efficiently, presumably due to their C-termini masking their NLSs. In order to gain insights into the nuclear transport of these proteins, we solved the crystal structures of importin-α bound to peptides corresponding to the supposed NLSs of MLH1 and PMS2 and performed isothermal titration calorimetry to study their binding affinities. Both putative MLH1 and PMS2 NLSs can bind to importin-α as monopartite NLSs, which is in agreement with some previous studies. However, MLH1-NLS has the highest affinity measured by a natural NLS peptide, suggesting a major role of MLH1 protein in nuclear import compared to PMS2. Finally, the role of MLH1 and PMS2 in the nuclear transport of the MutLα heterodimer is discussed.

MLH1和PMS2蛋白形成MutLα异二聚体，在人类DNA错配修复（MMR）中起主要作用。MMR相关蛋白中的突变与癌症有关，尤其是与结肠癌有关。MutLα的N末端区域包括PMS2和MLH1的N末端，同样，MutLα的C末端区域由PMS2和MLH1的C末端组成，二者通过接头区域相连。在该接头区域中发现了两种蛋白质的核转运所必需的核定位序列（NLS）。但是，确切的NLS序列引起争议，报道了不同的​​序列，特别是对于MLH1。各个组件的导入效率不高，可能是由于它们的C终端掩盖了NLS。为了深入了解这些蛋白质的核转运，我们解析了重要蛋白-α的晶体结构，该结构与假定的MLH1和PMS2的NLS对应的肽结合，并进行了等温滴定热法研究了它们的结合亲和力。推定的MLH1和PMS2 NLS都可以与importin-α结合为单部分NLS，这与以前的一些研究一致。然而，通过天然NLS肽测定，MLH1-NLS具有最高的亲和力，表明与PMS2相比，MLH1蛋白在核输入中起主要作用。最后，讨论了MLH1和PMS2在MutLα异二聚体的核转运中的作用。这与以前的一些研究是一致的。然而，通过天然NLS肽测定，MLH1-NLS具有最高的亲和力，表明与PMS2相比，MLH1蛋白在核输入中起主要作用。最后，讨论了MLH1和PMS2在MutLα异二聚体的核转运中的作用。这与以前的一些研究是一致的。然而，通过天然NLS肽测定，MLH1-NLS具有最高的亲和力，表明与PMS2相比，MLH1蛋白在核输入中起主要作用。最后，讨论了MLH1和PMS2在MutLα异二聚体的核转运中的作用。