Phosphorylation by Sky1p promotes Npl3p shuttling and mRNA dissociation

Mammalian SR proteins are currently thought to function in mRNA export as w ell as splicing. They contain multiple phosphorylated serine/arginine (RS/S R) dipeptides. Although SR domains can be phosphorylated by many kinases in vitro, the physiologically relevant kinase(s), and the role(s) of these mo difications in vivo have remained unclear. NpI3 is a shuttling protein in b udding 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 w e demonstrate that Sky1p phosphorylates only one of NpI3p's eight SR/RS dip eptides. Mutation of the C-terminal RS to RA, or deletion of SKY1, results in the cytoplasmic accumulation of NpI3p, The redistribution of Npl3p is ac companied by its increased association with poly(A)(+) RNA and decreased as sociation with its import receptor, Mtr10p, in vivo. We propose that phosph orylation of NpI3p by the cytoplasmically localized Sky1p is required for e fficient release of mRNA upon termination of export.