Hypophosphorylated ASF/SF2 binds TAP and is present in messenger ribonucleoproteins

Serine/arginine-rich proteins (SR proteins) function in precursor mRNA (pre-mRNA) splicing and may also act as adaptors for mRNA export. SR proteins are dynamically phosphorylated in their RS domain, and differential phosphorylation modulates their splicing activity and subcellular localization. In this study, we investigated the influence of phosphorylation on the function of SR proteins in events occurring during mRNA maturation. Immunoprecipitation experiments showed that the mRNA export receptor TAP associates preferentially with the hypophosphorylated form of shuttling SR proteins, including ASF/SF2. Overexpression of ASF induced subnuclear relocalization of TAP to SR protein-enriched nuclear speckles, suggesting their interaction in vivo. Moreover, the ASF found in a nucleoplasmic fraction rich in heterogeneous nuclear ribonucleoprotein (hnRNP) complexes is hyperphosphorylated, whereas mature messenger RNP (mRNP)-bound ASF is hypophosphorylated. Therefore, hypophosphorylation of ASF in mRNPs coincides with its higher affinity for TAP, suggesting that dephosphorylation of ASF promotes both its incorporation into mRNPs and recruitment of TAP for mRNA export. Thus, the phosphorylation state of RS domains may modulate the function of mammalian shuttling SR proteins during mRNA maturation or export.