Structural and calorimetric studies reveal specific determinants for the binding of a high-affinity NLS to mammalian importin-alpha.

The classical nuclear import pathway is mediated by importin (Impα and Impß), which recognizes the cargo protein by its nuclear localization sequence (NLS). NLSs have been extensively studied resulting in different proposed consensus; however, recent studies showed that exceptions may occur. This mechanism may be also dependent on specific characteristics of different Impα. Aiming to better understand the importance of specific residues from consensus and adjacent regions of NLSs, we studied different mutations of a high-affinity NLS complexed to Impα by crystallography and calorimetry. We showed that although the consensus sequence allows Lys or Arg residues at the second residue of a monopartite sequence, the presence of Arg is very important to its binding in major and minor sites of Impα. Mutations in the N or C-terminus (position P1 or P6) of the NLS drastically reduces their affinity to the receptor, which is corroborated by the loss of hydrogen bonds and hydrophobic interactions. Surprisingly, a mutation in the far N-terminus of the NLS led to an increase in the affinity for both binding sites, corroborated by the structure with an additional hydrogen bond. The binding of NLSs to the human variant Impα1 revealed that these are similar to those found in structures presented here. For human variant Impα3, the bindings are only relevant for the major site. This study increases understanding of specific issues sparsely addressed in previous studies that are important to the task of predicting NLSs, which will be relevant in the eventual design of synthetic NLSs.

Comparative study of the interactions between fungal transcription factor nuclear localization sequences with mammalian and fungal importin-alpha.

Importin-α (Impα) is an adaptor protein that binds to cargo proteins (containing Nuclear Localization Sequences - NLSs), for their translocation to the nucleus. The specificities of the Impα/NLS interactions have been studied, since these features could be used as important tools to find potential NLSs in nuclear proteins or even for the development of targets to inhibit nuclear import or to design peptides for drug delivery. Few structural studies have compared different Impα variants from the same organism or Impα of different organisms. Previously, we investigated nuclear transport of transcription factors with Neurospora crassa Impα (NcImpα). Herein, NIT-2 and PAC-3 transcription factors NLSs were studied in complex with Mus musculus Impα (MmImpα). Calorimetric assays demonstrated that the PAC-3 NLS peptide interacts with both Impα proteins with approximately the same affinity. The NIT-2 NLS sequence binds with high affinity to the Impα major binding site from both organisms, but its binding to minor binding sites reveals interesting differences due to the presence of additional interactions of NIT-2-NLS with MmImpα. These findings, together with previous results with Impα from other organisms, indicate that the differential affinity of NLSs to minor binding sites may be also responsible for the selectivity of some cargo proteins recognition and transport.

DNA mismatch repair proteins MLH1 and PMS2 can be imported to the nucleus by a classical nuclear import pathway.

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.

Structural basis of nuclear import of flap endonuclease 1 (FEN1).

Flap endonuclease 1 (FEN1) is a member of the nuclease family and is structurally conserved from bacteriophages to humans. This protein is involved in multiple DNA-processing pathways, including Okazaki fragment maturation, stalled replication-fork rescue, telomere maintenance, long-patch base-excision repair and apoptotic DNA fragmentation. FEN1 has three functional motifs that are responsible for its nuclease, PCNA-interaction and nuclear localization activities, respectively. It has been shown that the C-terminal nuclear localization sequence (NLS) facilitates nuclear localization of the enzyme during the S phase of the cell cycle and in response to DNA damage. To determine the structural basis of the recognition of FEN1 by the nuclear import receptor importin α, the crystal structure of the complex of importin α with a peptide corresponding to the FEN1 NLS was solved. Structural studies confirmed the binding of the FEN1 NLS as a classical bipartite NLS; however, in contrast to the previously proposed (354)KRKX(8)KKK(367) sequence, it is the (354)KRX(10)KKAK(369) sequence that binds to importin α. This result explains the incomplete inhibition of localization that was observed on mutating residues (365)KKK(367). Acidic and polar residues in the X(10) linker region close to the basic clusters play an important role in binding to importin α. These results suggest that the basic residues in the N-terminal basic cluster of bipartite NLSs may play roles that are more critical than those of the many basic residues in the C-terminal basic cluster.

The efficacy and safety of levobupivacaine administered by lumbosacral epidural route in conscious sheep.

The aim of this study was to evaluate 3 doses of levobupivacaine (LB) epidurally administered in sheep. Six adult male 24-36 month-old sheep received levobupivacaine at 3 doses, LB05 (0.05 mg/kg), LB15 (0.15 mg/kg), and LB25 (0.25mg/kg), and saline solution into the lumbosacral epidural space. Heart rate, arterial blood pressure (systolic, diastolic, and mean), respiratory rate, rectal, and skin temperature, local anesthesia, and ataxia were determined before treatment and at predetermined intervals. The duration of local anesthesia was 30±5 min, 145±27 min, and 290±18 min for LB05, LB15, and LB25, respectively (P<0.05). Ataxia determined for LB05, LB15, or LB25 was similar to the anesthetic times. There was an increase in heart rate and reduction in arterial pressure in LB25 (P<0.05), whereas LB05 or LB15 did not affect these parameters. Lumbosacral epidural levobupivacaine is an appropriate choice for local anesthesia in sheep.

Structural basis of importin-α-mediated nuclear transport for Ku70 and Ku80.

Ku70 and Ku80 form a heterodimeric complex involved in multiple nuclear processes. This complex plays a key role in DNA repair due to its ability to bind DNA double-strand breaks and facilitate repair by the nonhomologous end-joining pathway. Ku70 and Ku80 have been proposed to contain bipartite and monopartite nuclear localization sequences (NLSs), respectively, that allow them to be translocated to the nucleus independently of each other via the classical importin-α (Impα)/importin-ß-mediated nuclear import pathway. To determine the structural basis of the recognition of Ku70 and Ku80 proteins by Impα, we solved the crystal structures of the complexes of Impα with the peptides corresponding to the Ku70 and Ku80 NLSs. Our structural studies confirm the binding of the Ku80 NLS as a classical monopartite NLS but reveal an unexpected binding mode for Ku70 NLS with only one basic cluster bound to the receptor. Both Ku70 and Ku80 therefore contain monopartite NLSs, and sequences outside the basic cluster make favorable interactions with Impα, suggesting that this may be a general feature in monopartite NLSs. We show that the Ku70 NLS has a higher affinity for Impα than the Ku80 NLS, consistent with more extensive interactions in its N-terminal region. The prospect of nuclear import of Ku70 and Ku80 independently of each other provides a powerful regulatory mechanism for the function of the Ku70/Ku80 heterodimer and independent functions of the two proteins.