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First published online June 3, 2009
doi: 10.1242/10.1242/jcs.047373

Journal of Cell Science 122, 2100-2107 (2009)
Published by The Company of Biologists 2009
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Research Article

An integral protein of the inner nuclear membrane localizes to the mitotic spindle in mammalian cells

Charlotta Buch1,2,*, Robert Lindberg2,3,*, Ricardo Figueroa2,3, Santhosh Gudise1,2, Evgeny Onischenko4 and Einar Hallberg2,{ddagger}

1 Department of Biosciences and Nutrition, Karolinska Institute, SE-141 57 Huddinge, Sweden
2 Life Sciences, Södertörns University, SE-141 89 Huddinge, Sweden
3 Department of Biochemistry and Biophysics, Stockholm University, SE-106 91 Stockholm, Sweden
4 Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA

{ddagger} Author for correspondence (e-mail: einar.hallberg{at}sh.se)

Accepted 19 March 2009

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.

Key words: Inner nuclear membrane, Transmembrane proteins, Mitotic spindle, Mitosis, Cancer


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