The spatial targeting and nuclear matrix binding domains of SRm160.

The Ser-Arg (SR)-related protein SRm160 is a coactivator of pre-mRNA splicing. It bridges splicing factors located at the 5' splice site, branch site, and 3' splice site. Recently, SRm160 has also been shown to be involved in mRNA export as part of an exon-junction complex. SRm160 is highly concentrated in splicing speckles but is also present in long branched intranuclear tracks connecting splicing speckles with sites at the nuclear lamina. In this study we identified domains of SRm160 important for spatial targeting within the nucleus and for binding to the nuclear matrix. Using a series of FLAG- and enhanced GFP-conjugated deletion mutants we found two contiguous sequences that independently target SRm160 to nuclear matrix sites at splicing speckled domains: amino acids 300-350 and 351-688. Constructs containing amino acids 300-350 were also targeted to sites peripheral to speckled domains where most mRNA originate subsequent to splicing. Sequences from the N-terminal domain localized proteins to the nuclear lamina near sites where mRNA leaves the nucleus.