Nuclear import of human MLH1, PMS2, and MutLalpha: redundancy is the key.
DNA mismatch repair maintains genomic stability by correcting errors that have escaped polymerase proofreading. Defects on mismatch repair genes lead to an increased mutation rate, microsatellite instability and predisposition to human non-polyposis colorectal cancer (HNPCC). Human MutLalpha is a heterodimer formed by the interaction of MLH1 and PMS2 that coordinates a series of key events in mismatch repair. It has been proposed that nuclear import of MutLalpha may be the first regulatory step on the activation of the mismatch repair pathway. Using confocal microscopy and mismatch repair deficient cells, we have identified the sequence determinants that drive nuclear import of human MLH1, PMS2, and MutLalpha. Transient transfection of the individual proteins reveals that MLH1 has a bipartite and PMS2 has a single monopartite nuclear localization signal. Although dimerization is not required for nuclear localization, the MutLalpha heterodimer is imported more efficiently than the MLH1 or PMS2 monomers. Interestingly, the bipartite localization signal of MLH1 can direct import of MutLalpha even when PMS2 encompasses a mutated localization signal. Hence we conclude that the presence of redundant nuclear localization signals guarantees nuclear transport of MutLalpha and, consequently, efficient mismatch repair.
Leong
Vivian
V
Department of Biochemistry and Biomedical Sciences, McMaster University, Ontario, Canada.
Lorenowicz
Jessica
J
Kozij
Natalie
N
Guarné
Alba
A
D021581:Active Transport, Cell Nucleus; D048868:Adaptor Proteins, Signal Transducing / Q000235:genetics / Q000378:metabolism*; D000230:Adenocarcinoma / Q000235:genetics / Q000378:metabolism; D000251:Adenosine Triphosphatases / Q000235:genetics / Q000378:metabolism*; D002467:Cell Nucleus / Q000378:metabolism*; D015179:Colorectal Neoplasms / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D045643:DNA Repair Enzymes / Q000235:genetics / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D005455:Fluorescent Antibody Technique; D006801:Humans; D000070976:Mismatch Repair Endonuclease PMS2; D000070957:MutL Protein Homolog 1; D000070956:MutL Proteins; D019913:Nuclear Localization Signals; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism*; D014162:Transfection
D048868:Adaptor Proteins, Signal Transducing; D004268:DNA-Binding Proteins; C501486:MLH1 protein, human; C500929:MutLalpha protein, human; D019913:Nuclear Localization Signals; D009687:Nuclear Proteins; D000251:Adenosine Triphosphatases; C094934:PMS2 protein, human; D000070976:Mismatch Repair Endonuclease PMS2; D000070957:MutL Protein Homolog 1; D000070956:MutL Proteins; D045643:DNA Repair Enzymes