Isoformy proteínu myosin 1c a jejich funkce v buněčném jádře a v cytoplasmě

Nuclear myosin 1 (NM1) was the first myosin described in the cell nucleus. From its discovery, it has been found to function in processes of Pol I and Pol II transcription, chromatin remodeling, and chromosomal movements. However, direct mechanisms of how NM1 works in the cell nucleus were still missing. We therefore decided to prepare NM1 knock-out mice to answer questions about phyiological functioning of this protein. Myo1c is an isoform of NM1 protein, previously described in the cytoplasm. The only difference between these isoforms is 16 amino-acids at the N-terminus of NM1, which were thought to be the nuclear localization signal. However, we discovered that the nuclear localization signal is located in the neck domain of myosin, and therefore it is able to direct both isoforms to the nucleus. Moreover, we found that the ratio between both proteins is nearly the same in the nucleus and deletion of NM1 does not cause compensatory overexpression of Myo1c. NM1 KO mice are fully viable with minor changes in bone mineral density and red blood cells size. We found that the function of NM1 in processes such as Pol I transcription can be fully covered by Myo1c protein, suggesting redundancy and interchangeability of these two isoforms in the cell nucleus. We also found that PIP2, a phosphoinositol...Souhrn Jaderný myosin 1 je první myosin, který byl nalezený v buněčném jádře. Účastní se procesů jako je transkripce polymerasami I a II, remodelace chromatinu a pohyby chromosomů. Dosud však nebyl popsán přesný mechanismus funkce NM1 v buněčném jádře. Proto jsme připravili myš, která má deleci v genu kódujícím NM1 (NM1 KO). Myo1c je izoforma NM1, která byla charakterizována jako cytoplasmatická. Jediným rozdílem mezi těmito dvěma izoformami je 16 aminokyselin, které obsahuje NM1 na svém N-konci a které byly dříve považovány za jaderný lokalizační signál. Nám se ale podařilo ukázat, že obě izoformy jsou translokovány do jádra, protože jaderný lokalizační signál se nachází v krku, což je doména, která je společná pro obě izoformy. Navíc jsme ukázali, že poměr mezi oběma izoformami je v jádře i v cytoplasmě stejný a delece NM1 nezpůsobuje kompenzační expresi Myo1c. NM1 KO myši jsou životaschopné a vykazují minoritní změny v minerální hustotě kostí a velikosti červených krvinek. Dále jsme zjistili, že Myo1c může NM1 zastoupit ve funkcích jako je transkripce polymerasou I, což naznačuje, že tyto dvě izoformy jsou v buněčném jádře zaměnitelné a duplicitní. V cytoplasmě byla popsána interakce mezi Myo1c a PIP2, lipidem ze skupiny fosfoinositolů. Ukázali jsme, že PIP2 se nachází i v jádře, kde přispívá k propojení...Department of Cell BiologyKatedra buněčné biologieFaculty of SciencePřírodovědecká fakult

Mouse nuclear myosin I knock-out shows interchangeability and redundancy of myosin isoforms in the cell nucleus.

Nuclear myosin I (NM1) is a nuclear isoform of the well-known "cytoplasmic" Myosin 1c protein (Myo1c). Located on the 11(th) chromosome in mice, NM1 results from an alternative start of transcription of the Myo1c gene adding an extra 16 amino acids at the N-terminus. Previous studies revealed its roles in RNA Polymerase I and RNA Polymerase II transcription, chromatin remodeling, and chromosomal movements. Its nuclear localization signal is localized in the middle of the molecule and therefore directs both Myosin 1c isoforms to the nucleus. In order to trace specific functions of the NM1 isoform, we generated mice lacking the NM1 start codon without affecting the cytoplasmic Myo1c protein. Mutant mice were analyzed in a comprehensive phenotypic screen in cooperation with the German Mouse Clinic. Strikingly, no obvious phenotype related to previously described functions has been observed. However, we found minor changes in bone mineral density and the number and size of red blood cells in knock-out mice, which are most probably not related to previously described functions of NM1 in the nucleus. In Myo1c/NM1 depleted U2OS cells, the level of Pol I transcription was restored by overexpression of shRNA-resistant mouse Myo1c. Moreover, we found Myo1c interacting with Pol II. The ratio between Myo1c and NM1 proteins were similar in the nucleus and deletion of NM1 did not cause any compensatory overexpression of Myo1c protein. We observed that Myo1c can replace NM1 in its nuclear functions. Amount of both proteins is nearly equal and NM1 knock-out does not cause any compensatory overexpression of Myo1c. We therefore suggest that both isoforms can substitute each other in nuclear processes

Specific Nuclear Localizing Sequence Directs Two Myosin Isoforms to the Cell Nucleus in Calmodulin-Sensitive Manner

BACKGROUND: Nuclear myosin I (NM1) was the first molecular motor identified in the cell nucleus. Together with nuclear actin, they participate in crucial nuclear events such as transcription, chromatin movements, and chromatin remodeling. NM1 is an isoform of myosin 1c (Myo1c) that was identified earlier and is known to act in the cytoplasm. NM1 differs from the "cytoplasmic" myosin 1c only by additional 16 amino acids at the N-terminus of the molecule. This amino acid stretch was therefore suggested to direct NM1 into the nucleus. METHODOLOGY/PRINCIPAL FINDINGS: We investigated the mechanism of nuclear import of NM1 in detail. Using over-expressed GFP chimeras encoding for truncated NM1 mutants, we identified a specific sequence that is necessary for its import to the nucleus. This novel nuclear localization sequence is placed within calmodulin-binding motif of NM1, thus it is present also in the Myo1c. We confirmed the presence of both isoforms in the nucleus by transfection of tagged NM1 and Myo1c constructs into cultured cells, and also by showing the presence of the endogenous Myo1c in purified nuclei of cells derived from knock-out mice lacking NM1. Using pull-down and co-immunoprecipitation assays we identified importin beta, importin 5 and importin 7 as nuclear transport receptors that bind NM1. Since the NLS sequence of NM1 lies within the region that also binds calmodulin we tested the influence of calmodulin on the localization of NM1. The presence of elevated levels of calmodulin interfered with nuclear localization of tagged NM1. CONCLUSIONS/SIGNIFICANCE: We have shown that the novel specific NLS brings to the cell nucleus not only the "nuclear" isoform of myosin I (NM1 protein) but also its "cytoplasmic" isoform (Myo1c protein). This opens a new field for exploring functions of this molecular motor in nuclear processes, and for exploring the signals between cytoplasm and the nucleus

Myosin 1c isoforms and their functions in the cell nucleus and in the cytoplasm

Nuclear myosin 1 (NM1) was the first myosin described in the cell nucleus. From its discovery, it has been found to function in processes of Pol I and Pol II transcription, chromatin remodeling, and chromosomal movements. However, direct mechanisms of how NM1 works in the cell nucleus were still missing. We therefore decided to prepare NM1 knock-out mice to answer questions about phyiological functioning of this protein. Myo1c is an isoform of NM1 protein, previously described in the cytoplasm. The only difference between these isoforms is 16 amino-acids at the N-terminus of NM1, which were thought to be the nuclear localization signal. However, we discovered that the nuclear localization signal is located in the neck domain of myosin, and therefore it is able to direct both isoforms to the nucleus. Moreover, we found that the ratio between both proteins is nearly the same in the nucleus and deletion of NM1 does not cause compensatory overexpression of Myo1c. NM1 KO mice are fully viable with minor changes in bone mineral density and red blood cells size. We found that the function of NM1 in processes such as Pol I transcription can be fully covered by Myo1c protein, suggesting redundancy and interchangeability of these two isoforms in the cell nucleus. We also found that PIP2, a phosphoinositol..

Focal Adhesion Protein Vinculin Is Required for Proper Meiotic Progression during Mouse Spermatogenesis

The focal adhesion protein Vinculin (VCL) is ascribed to various cytoplasmic functions; however, its nuclear role has so far been ambiguous. We observed that VCL localizes to the nuclei of mouse primary spermatocytes undergoing first meiotic division. Specifically, VCL localizes along the meiosis-specific structure synaptonemal complex (SC) during prophase I and the centromeric regions, where it remains until metaphase I. To study the role of VCL in meiotic division, we prepared a conditional knock-out mouse (VCLcKO). We found that the VCLcKO male mice were semi-fertile, with a decreased number of offspring compared to wild-type animals. This study of events in late prophase I indicated premature splitting of homologous chromosomes, accompanied by an untimely loss of SCP1. This caused erroneous kinetochore formation, followed by failure of the meiotic spindle assembly and metaphase I arrest. To assess the mechanism of VCL involvement in meiosis, we searched for its possible interacting partners. A mass spectrometry approach identified several putative interactors which belong to the ubiquitin–proteasome pathway (UPS). The depletion of VLC leads to the dysregulation of a key subunit of the proteasome complex in the meiotic nuclei and an altered nuclear SUMOylation level. Taken together, we show for the first time the presence of VCL in the nucleus of spermatocytes and its involvement in proper meiotic progress. It also suggests the direction for future studies regarding the role of VCL in spermatogenesis through regulation of UPS

Identification of NM1 interacting proteins in the cytosol.

<p>Digitonin extract from suspension HeLa cells was incubated with recombinant Str-IQ12-His peptide containing N-terminal OneStrep tag (IQ12) and Streptactin beads as a control for background binding. Bound proteins were resolved on 4–20% SDS-PAGE gel and stained with SimplyBlue. Mass spectrometric analysis of the protein bands that co-purified with bait (arrows) identified importin 5 and heat shock protein 90 beta (HSP90) (<b>A</b>). SimplyBlue stained 4–20% SDS-PAGE gel with proteins that interacted with Str-GFP-NM1-(Q123.T) and Str-GFP as a control in digitonin extract of HEK293T cells. The arrows show positions of bands that contained proteins identified using mass spectrometry as importin 5, importin 7, importin-β1, HSP90 beta and calmodulin (<b>B</b>). Proteins that co-immunoprecipitate with antibody to endogenous NM1 from HeLa extracts were resolved using SDS-PAGE and tranferred onto nitrocelulose membrane. Membrane was probed with with anti-NM1, anti-importin 5 (IPO5), anti-importin 7 (IPO7), anti-importin-β1 (KPNB1). Rabbit polyclonal antibody against GFP was used as a control for backgroung binding (<b>C</b>). N-terminally Strep tagged GFP-NM1-(Q123.T) ^NLSwt (wt), GFP-NM1-(Q123.T) ^NLSmut (mut) and GFP as negative control (nc) were expressed in HEK293T cells. Cells were extracted with buffer containing digitonin (digi) to obtain soluble cytosol; pellet was re-extracted with the same buffer containing 1% Triton X-100 (triton). Bound proteins were resolved on SDS-PAGE, transferred to nitrocelulose. Membrane was incubated with antibody to importin 5 ans GFP (<b>D</b>). Beads containing Str-GFP-NM1-(Q123.T) and Str-GFP-SV40 NLS and associated proteins were eluted first with buffer containing GTP-loaded RanQ69L or buffer alone and then with biotin containig buffer that liberated Strep-tagged bait proteins from the column. Proteins eluted from the beads were resolved on SDS-PAGE and transferred to nitrocelulose membrane. GFP, importin 5 and importin-β1 signals were detected using specific antibodies (<b>E</b>). Signal from secondary antibodies was detected using LI-COR Odyssey infrared imaging system.</p

Mutation of basic residues in the neck of NM1/Myo1c abolishes its nuclear import.

<p>U2OS cells were transfected with full length NM1-V5/His (<b>A</b>), NM1-V5/His lacking the second IQ motif (<b>B</b>), and NM1-V5/His with point mutation of basic amino acids within the NLS into alanines (<b>C</b>). Below the pictures are schematic representations of constructs used. Color coding is the same as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030529#pone-0030529-g002" target="_blank"><b>Fig. 2</b></a>. Cells were fixed 48 hours post transfection and labeled with anti-V5 antibody, pictures were obtained using wide-field microscope, scale bar: 10 µm (<b>D</b>) U2OS cells transiently transfected with Myo1c-V5/His show nuclear localization of the protein Picture is a single confocal plane, obtained by confocal microscope. Scale bar: 10 µm. (<b>E</b>) Nuclear and cytosolic extracts were prepared from liver of either wild type (WT) or NM1 knock-out (KO) mice. Equal amount of protein was resolved using SDS-PAGE and electro-transferred to nitrocellulose. Membrane was probed with anti-NM1, anti-Myo1c, anti hnRNP C1/C2 and GAPDH antibody. Signal was detected using LI-COR Odyssey infrared imaging system. (<b>F</b>) U2OS cells were transiently transfected with Myo1c-V5/His. 24 hours after transfection cells were treated with nocodazole or aphidicolin to stall the cells either in G2/M or in G1/S phase of cell cycle. After the release from the block cells were cultivated for another 24 hours. Samples were taken in indicated timepoints. Cells were labeled with antibody to V5 tag, patterns counted and divided into three groups according to the localization of fluorescent proteins. More than 100 cells were counted in each timepoint, expreriment was repeated twice with similar result.</p

Neck domain of NM1 contains the NLS.

<p>U2OS cell transfected with a panel of truncation constructs of full length NM1 (<b>A–F</b>) and IQ domains fused to GFP-PK (<b>G–L</b>). Cells were fixed 48 hours post transfection. Below the pictures are schematic representations of the truncations affecting various NM1 domains as well as the GFP-PK phusions. Pictures (<b>A–F</b>) were acquired using confocal microscope, single confocal planes are shown. Pictures (<b>G–L</b>) were photographed using wide-field fluorescent microscope. Scale bar: 10 µm.</p