Journal article Open Access

Alternatively spliced N-terminal exons in tropomyosin isoforms do not act as autonomous targeting signals

Martin, Claire; Schevzov, Galina; Gunning, Peter

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    <subfield code="a">Tropomyosin (Tm) polymerises head-to-tail to form a continuous polymer located in the major groove of
the actin filament. Multiple Tm isoforms are generated by alternative splicing of four genes, and individual
isoforms show specific localisation patterns in many cell types, and can have differing effects on the
actin cytoskeleton. Fluorescently-tagged Tm isoforms and mutants were expressed in C2C12 cells to
investigate the mechanisms of alternative localisation of high molecular weight (HMW) and low molecular
weight (LMW) Tms. Fluorescently-tagged Tm constructs show similar localisation to endogenous
Tms as observed by antibodies, with the HMW Tm3 relatively diminished at the periphery of cells compared
to LMW isoforms Tm5b or Tm5NM1. Tm3 and Tm5b only differ in their N-terminal exons, but these
N-terminal exons do not independently direct localisation within the cell, as chimeric mutants Tm3–
Tm5NM1 and Tm5b–Tm5NM1 show an increased peripheral localisation similar to Tm5NM1. The lower
abundance of Tm3 at the periphery of the cell is not a result of different protein dynamics, as Tm3 and
Tm5b show similar recovery after photobleaching. The relative exclusion of Tm3 from the periphery of
cells does, however, require interaction with the actin filament, as mutants with truncations at either
the N-terminus or the C-terminus are unable to localise to actin stress fibres, and are present in the most
peripheral regions of the cell. We conclude that it is the entire Tm molecule which is the unit of sorting,
and that the alternatively spliced N-terminal exons do not act as autonomous targeting signals.</subfield>
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    <subfield code="a">Schevzov, Galina</subfield>
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    <subfield code="a">Gunning, Peter</subfield>
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    <subfield code="a">Martin, Claire</subfield>
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    <subfield code="a">10.1016/j.jsb.2009.12.016</subfield>
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    <subfield code="a">Alternatively spliced N-terminal exons in tropomyosin isoforms do not act as autonomous targeting signals</subfield>
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