Lamin A rod domain mutants target heterochromatin protein 1alpha and beta for proteasomal degradation by activation of F-box protein, FBXW10.
BACKGROUND
Lamins are major structural proteins of the nucleus and contribute to the organization of various nuclear functions. Mutations in the human lamin A gene cause a number of highly degenerative diseases, collectively termed as laminopathies. Cells expressing lamin mutations exhibit abnormal nuclear morphology and altered heterochromatin organization; however, the mechanisms responsible for these defects are not well understood.


RESULTS
The lamin A rod domain mutants G232E, Q294P and R386K are either diffusely distributed or form large aggregates in the nucleoplasm, resulting in aberrant nuclear morphology in various cell types. We examined the effects of these lamin mutants on the distribution of heterochromatin protein 1 (HP1) isoforms. HeLa cells expressing these mutants showed a heterogeneous pattern of HP1alpha and beta depletion but without altering HP1gamma levels. Changes in HP1alpha and beta were not observed in cells expressing wild-type lamin A or mutant R482L, which assembled normally at the nuclear rim. Treatment with proteasomal inhibitors led to restoration of levels of HP1 isoforms and also resulted in stable association of lamin mutants with the nuclear periphery, rim localization of the inner nuclear membrane lamin-binding protein emerin and partial improvement of nuclear morphology. A comparison of the stability of HP1 isoforms indicated that HP1alpha and beta displayed increased turnover and higher basal levels of ubiquitination than HP1gamma. Transcript analysis of components of the ubiquitination pathway showed that a specific F-box protein, FBXW10 was induced several-fold in cells expressing lamin mutants. Importantly, ectopic expression of FBXW10 in HeLa cells led to depletion of HP1alpha and beta without alteration of HP1gamma levels.


CONCLUSIONS
Mislocalized lamins can induce ubiquitin-mediated proteasomal degradation of certain HP1 isoforms by activation of FBXW10, a member of the F-box family of proteins that is involved in E3 ubiquitin ligase activity.
Chaturvedi
Pankaj
P
Centre for Cellular and Molecular Biology (CSIR), Hyderabad, India.
Parnaik
Veena K
VK
D015415:Biomarkers / Q000378:metabolism; D000090266:Chromobox Protein Homolog 5; D002868:Chromosomal Proteins, Non-Histone / Q000378:metabolism*; D044783:F-Box Proteins / Q000378:metabolism*; D006367:HeLa Cells; D006657:Histones / Q000378:metabolism; D006801:Humans; D034904:Lamin Type A / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D007976:Leupeptins / Q000494:pharmacology; D008565:Membrane Proteins / Q000378:metabolism; D008745:Methylation / Q000187:drug effects; D050505:Mutant Proteins / Q000737:chemistry / Q000378:metabolism*; D009685:Nuclear Envelope / Q000187:drug effects / Q000378:metabolism; D009687:Nuclear Proteins / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D061988:Proteasome Inhibitors; D020033:Protein Isoforms / Q000378:metabolism; D011499:Protein Processing, Post-Translational / Q000187:drug effects; D055550:Protein Stability / Q000187:drug effects; D017434:Protein Structure, Tertiary; D021381:Protein Transport / Q000187:drug effects; D011506:Proteins / Q000378:metabolism*; D012995:Solubility / Q000187:drug effects; D014162:Transfection
D015415:Biomarkers; C000715594:CBX5 protein, human; C109561:CDRT1 protein, human; D002868:Chromosomal Proteins, Non-Histone; D044783:F-Box Proteins; C574378:FBXW10 protein, human; D006657:Histones; D034904:Lamin Type A; D007976:Leupeptins; D008565:Membrane Proteins; D050505:Mutant Proteins; D009687:Nuclear Proteins; D061988:Proteasome Inhibitors; D020033:Protein Isoforms; D011506:Proteins; C092710:emerin; D000090266:Chromobox Protein Homolog 5; D046988:Proteasome Endopeptidase Complex; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde