Mutually exclusive interactions drive handover of mRNA from export adaptors to TAP

Adaptor proteins stimulate the nuclear export of mRNA, but their mechanism of action remains unclear. Here, we show that REF/ALY binds mRNA; but upon formation of a ternary complex with TAP the RNA is transferred from REF to TAP, and overexpression of TAP displaces REF from mRNA in vivo. RNA is also handed over from two other adaptors, 9G8 and SRp20 to TAP upon formation of a ternary complex. Interestingly, the RNA-binding affinity of TAP is enhanced 4-fold in vitro once it is complexed with REF. 9G8 and SRp20 also enhance the TAP RNA-binding activity in vitro. Consistent with a model in which TAP directly binds mRNA handed over from adaptors during export, we show that TAP binds mRNA in vivo by an arginine-rich motif in its N-terminal domain. The importance of direct TAP-mRNA interactions is confirmed by the observation that a mutant form of TAP that fails to bind mRNA but retains the ability to bind REF does not function in mRNA export.