The C terminus of mitosin is essential for its nuclear localization, centromere/kinetochore targeting, and dimerization

The cloning and analysis of LEK1 identifies variations in the LEK/centromere protein F/mitosin gene family

We report the cloning of a novel murine cDNA, LEK1, that is related to human CENP-F and mitosin and more distantly to chicken CMF1. The proteins from these three organisms have significant homology, yet differ in their temporal, spatial, and subcellular localizations. The human proteins bind the kinetochore in mitotic cells, whereas the chicken protein is found only in skeletal and cardiac muscle and is developmentally regulated. Mouse LEK1 is a single copy gene that codes for two developmentally regulated transcripts. The LEK1 protein is expressed early and ubiquitously in mouse development and is generally down-regulated as development proceeds in a manner that correlates to a cessation of mitosis. In adult tissues, the LEK1 protein is detected exclusively in the pronucleus of the oocyte and was not observed in other actively dividing tissues. Subcellular localization revealed that the LEK1 protein in mitotic cells does not bind the kinetochore. From these data, we hypothesize that chicken CMF1, human CENP-F, mitosin, and mouse LEK1 are members of an emerging family of genes that have important and functionally distinct roles in development and cell division.

Deciphering the nuclear import pathway for the cytoskeletal red cell protein 4.1R

The erythroid membrane cytoskeletal protein 4.1 is the prototypical member of a genetically and topologically complex family that is generated by combinatorial alternative splicing pathways and is localized at diverse intracellular sites including the nucleus. To explore the molecular determinants for nuclear localization, we transfected COS-7 cells with epitope-tagged versions of natural red cell protein 4.1 (4.1R) isoforms as well as mutagenized and truncated derivatives. Two distant topological sorting signals were required for efficient nuclear import of the 4.1R80 isoform: a basic peptide, KKKRER, encoded by alternative exon 16 and acting as a weak core nuclear localization signal (4.1R NLS), and an acidic peptide, EED, encoded by alternative exon 5. 4.1R80 isoforms lacking either of these two exons showed decreased nuclear import. Fusion of various 4.1R80 constructs to the cytoplasmic reporter protein pyruvate kinase confirmed a requirement for both motifs for full NLS function. 4.1R80 was efficiently imported in the nuclei of digitonin-permeabilized COS-7 cells in the presence of recombinant Rch1 (human importin alpha2), importin beta, and GTPase Ran. Quantitative analysis of protein-protein interactions using a resonant mirror detection technique showed that 4.1R80 bound to Rch1 in vitro with high affinity (KD = 30 nM). The affinity decreased at least 7- and 20-fold, respectively, if the EED motif in exon 5 or if 4.1R NLS in exon 16 was lacking or mutated, confirming that both motifs were required for efficient importin-mediated nuclear import of 4.1R80.

Structural requirements and dynamics of mitosin-kinetochore interaction in M phase

Mitosin is a 350-kDa human nuclear protein which transiently associates with centromeres and spindle poles in M phase. Ultrastructure studies reveal that it is located at the outer kinetochore plate. In this work, we explored the detailed structural basis and dynamics of the mitosin-kinetochore interaction. Two major regions important for targeting to centromeres were identified by analyzing different deletion mutants expressed in CHO cells: (i) the "core region" between amino acids 2792 and 2887, which was essential for the centromere localization of mitosin; and (ii) the internal repeats between residues 2094 and 2487, which cooperated with the core region to achieve strong mitosin-kinetochore interaction. The core region is characteristic of two leucine zipper motifs. Deletion of either motif abolished the centromere localization activity. In addition, Cys2864, adjacent to the second motif, was also essential for the activity of the core region. In contrast, the internal repeats alone were insufficient for centromere localization. We propose that this region may serve as a regulatory domain to facilitate interaction of the core region with the kinetochore. We showed that mitosin molecules entering nuclei after nuclear envelope breakdown (NEBD) were not assembled onto kinetochores efficiently, suggesting that the mitosin-kinetochore interaction is stabilized prior to NEBD. This result supports the idea of an ordered process for kinetochore assembly. Our data also suggest that mitosin might interact with chromatin in interphase. Evidence for coordinated regulation between the centromere-targeting and the putative chromatin-binding activities is also provided.

INCENP centromere and spindle targeting: identification of essential conserved motifs and involvement of heterochromatin protein HP1

The inner centromere protein (INCENP) has a modular organization, with domains required for chromosomal and cytoskeletal functions concentrated near the amino and carboxyl termini, respectively. In this study we have identified an autonomous centromere- and midbody-targeting module in the amino-terminal 68 amino acids of INCENP. Within this module, we have identified two evolutionarily conserved amino acid sequence motifs: a 13-amino acid motif that is required for targeting to centromeres and transfer to the spindle, and an 11-amino acid motif that is required for transfer to the spindle by molecules that have targeted previously to the centromere. To begin to understand the mechanisms of INCENP function in mitosis, we have performed a yeast two-hybrid screen for interacting proteins. These and subsequent in vitro binding experiments identify a physical interaction between INCENP and heterochromatin protein HP1(Hsalpha). Surprisingly, this interaction does not appear to be involved in targeting INCENP to the centromeric heterochromatin, but may instead have a role in its transfer from the chromosomes to the anaphase spindle.

Identification of a novel cytoplasmic protein that specifically binds to nuclear localization signal motifs

Active transport of proteins into the nucleus is mediated by interaction between the classical nuclear localization signals (NLSs) of the targeted proteins and the NLS receptor (importin) complex. This nuclear transport system is highly regulated and conserved in eukaryotes and is essential for cell survival. Using a fragment of BRCA1 containing the two NLS motifs as a bait for yeast two-hybrid screening, we have isolated four clones, one of which is importin alpha. Here we characterize one of the other clones identified, BRAP2, which is a novel gene and expressed as a 2-kilobase mRNA in human mammary epithelial cells and some but not all tissues of mice. The isolated full-length cDNA encodes a novel protein containing 600 amino acid residues with pI 6.04. Characteristic motifs of C2H2 zinc fingers and leucine heptad repeats are present in the middle and C-terminal regions of the protein, respectively. BRAP2 also shares significant homology with a hypothetical protein from yeast Saccharomyces cerevisiae, especially in the zinc finger region. Antibodies prepared against the C-terminal region of BRAP2 fused to glutathione S-transferase specifically recognize a cellular protein with a molecular size of 68 kDa, consistent with the size of the in vitro translated protein. Cellular BRAP2 is mainly cytoplasmic and binds to the NLS motifs of BRCA1 with similar specificity to that of importin alpha in both two-hybrid assays in yeast and glutathione S-transferase pull-down assays in vitro. Other motifs such as the SV40 large T antigen NLS motif and the bipartite NLS motif found in mitosin are also recognized by BRAP2. Similarly, the yeast homolog of BRAP2 also binds to these NLS motifs in vitro. These results imply that BRAP2 may function as a cytoplasmic retention protein and play a role in regulating transport of nuclear proteins.

Dynamic continuity of nuclear and mitotic matrix proteins in the cell cycle

The eukaryotic cell nucleus is a membrane-enclosed compartment containing the genome and associated molecules supported by a highly insoluble filamentous network known as the nucleoskeleton or nuclear matrix. The nuclear matrix is believed to play roles in maintaining nuclear architecture and organizing nuclear metabolism. Recently, advances in microscopic techniques and the availability of new molecular probes have made it possible to localize functional domains within the nuclear matrix and demonstrate dynamic interactions between both soluble and insoluble components involved in the control of multiple nuclear transactions. Like the cytoplasm and its skeleton, the nucleoplasm is highly structured and very crowded with an equally complex skeletal framework. In fact, there is growing evidence that the two skeletal systems are functionally contiguous, providing a dynamic cellular matrix connecting the cell surface with the genome. If we impose cell cycle dynamics upon this skeletal organization, it is obvious that the genome and associated nuclear matrix must undergo a major structural transition during mitosis, being disassembled and/or reorganized in late G2 and reassembled again in daughter nuclei. However, recent evidence from our laboratory and elsewhere suggests that much of the nuclear matrix is used to form the mitotic apparatus (MA). Indeed, both facultative and constitutive matrix-associated proteins such as NuMA, CENP-B, CENP-F, and the retinoblastoma protein (Rb) associate within and around the MA. During mitosis, the nuclear matrix proteins may either become inert "passengers" or assume critical functions in partitioning the genome into newly formed G1 nuclei. Therefore, we support the view that the nuclear matrix exists as a dynamic architectural continuum, embracing the genome and maintaining cellular regulation throughout the cell cycle.