title: An integral protein of the inner nuclear membrane localizes to the mitotic spindle in mammalian cells.
abstract: Here, we characterize a transmembrane protein of the nuclear envelope that we name spindle-associated membrane protein 1 (Samp1). The protein is conserved in metazoa and fission yeast and is homologous to Net5 in rat and Ima1 in Schizosaccharomyces pombe. We show that, in human cells, the protein is a membrane-spanning polypeptide with an apparent molecular mass of 43 kDa. This is consistent with a predicted polypeptide of 392 amino acids that has five transmembrane segments and its C-terminus exposed to the nucleoplasm. During interphase, Samp1 was specifically distributed in the inner nuclear membrane. Post-transcriptional silencing of Samp1 expression resulted in separation of centrosomes from the nuclear envelope, indicating that it is functionally connected to the cytoskeleton. At the onset of mitosis, most of the protein dispersed out into the ER, as expected. However, during mitosis, a significant fraction of the protein specifically localized to the polar regions of the mitotic spindle. We demonstrate for the first time, in human cells, the existence of a membranous structure overlapping with the mitotic spindle. Interestingly, another integral inner nuclear membrane protein, emerin, was absent from the spindle-associated membranes. Thus, Samp1 defines a specific membrane domain associated with the mitotic spindle.
authors.affiliation: Department of Biosciences and Nutrition, Karolinska Institute, SE-141 57 Huddinge, Sweden.
authors.forename: Charlotta
authors.initials: C
authors.lastname: Buch
authors.forename: Robert
authors.initials: R
authors.lastname: Lindberg
authors.forename: Ricardo
authors.initials: R
authors.lastname: Figueroa
authors.forename: Santhosh
authors.initials: S
authors.lastname: Gudise
authors.forename: Evgeny
authors.initials: E
authors.lastname: Onischenko
authors.forename: Einar
authors.initials: E
authors.lastname: Hallberg
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002478:Cells, Cultured; D004285:Dogs; D006367:HeLa Cells; D006801:Humans; D008322:Mammals; D008565:Membrane Proteins / Q000378:metabolism*; D008954:Models, Biological; D008969:Molecular Sequence Data; D009685:Nuclear Envelope / Q000378:metabolism*; D009687:Nuclear Proteins / Q000378:metabolism; D010802:Phylogeny; D021381:Protein Transport; D017386:Sequence Homology, Amino Acid; D008941:Spindle Apparatus / Q000378:metabolism*
chemical_list: D008565:Membrane Proteins; D009687:Nuclear Proteins