Mammalian SR proteins are currently thought to function
in mRNA export as well as splicing. They contain multiple
phosphorylated serine/arginine (RS/SR) dipeptides.
Although SR domains can be phosphorylated by many kinases
in vitro, the physiologically relevant kinase(s), and the
role(s) of these modifications in vivo have remained unclear.
Npl3 is a shuttling protein in budding yeast that we showed
previously to be a substrate for the mammalian SR protein
kinase, SRPK1, as well as the related yeast kinase, Sky1.
Here we demonstrate that Sky1p phosphorylates only one
of Npl3p's eight SR/RS dipeptides. Mutation of the
C-terminal RS to RA, or deletion of SKY1, results in the
cytoplasmic accumulation of Npl3p. The redistribution of
Npl3p is accompanied by its increased association with
poly(A)+ RNA and decreased association with
its import receptor, Mtr10p, in vivo. We propose that phosphorylation
of Npl3p by the cytoplasmically localized Sky1p is required
for efficient release of mRNA upon termination of export.