Both Sm-domain and C-terminal extension of Lsm1 are important for the RNA-binding activity of the Lsm1–7–Pat1 complex

  1. Sundaresan Tharun2
  1. Department of Biochemistry, Uniformed Services University of the Health Sciences (USUHS), Bethesda, Maryland 20814-4799, USA
    • 1 Present address: Department of Biotechnology, Gokaraju Rangaraju Institute of Engineering and Technology (GRIET), Bachupally, Kukatpally, Hyderabad 500090, India.

    Abstract

    Lsm proteins are a ubiquitous family of proteins characterized by the Sm-domain. They exist as hexa- or heptameric RNA-binding complexes and carry out RNA-related functions. The Sm-domain is thought to be sufficient for the RNA-binding activity of these proteins. The highly conserved eukaryotic Lsm1 through Lsm7 proteins are part of the cytoplasmic Lsm1–7–Pat1 complex, which is an activator of decapping in the conserved 5′–3′ mRNA decay pathway. This complex also protects mRNA 3′-ends from trimming in vivo. Purified Lsm1–7–Pat1 complex is able to bind RNA in vitro and exhibits a unique binding preference for oligoadenylated RNA (over polyadenylated and unadenylated RNA). Lsm1 is a key subunit that determines the RNA-binding properties of this complex. The normal RNA-binding activity of this complex is crucial for mRNA decay and 3′-end protection in vivo and requires the intact Sm-domain of Lsm1. Here, we show that though necessary, the Sm-domain of Lsm1 is not sufficient for the normal RNA-binding ability of the Lsm1–7–Pat1 complex. Deletion of the C-terminal domain (CTD) of Lsm1 (while keeping the Sm-domain intact) impairs mRNA decay in vivo and results in Lsm1–7–Pat1 complexes that are severely impaired in RNA binding in vitro. Interestingly, the mRNA decay and 3′-end protection defects of such CTD-truncated lsm1 mutants could be suppressed in trans by overexpression of the CTD polypeptide. Thus, unlike most Sm-like proteins, Lsm1 uniquely requires both its Sm-domain and CTD for its normal RNA-binding function.

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    Footnotes

    • Received August 13, 2011.
    • Accepted February 24, 2012.
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