SRAG Functions as a New mRNA Export Co-adaptor

DEAD box RNA helicases play important roles in many cellular processes including splicing, mRNA export and translation. The two domains of many DEAD box RNA helicases adopt radically different conformations according to whether they are nucleotide free, ADP or ATP bound. This change in conformation is often harnessed to drive subunit rearrangements in multiprotein complexes. Assembly of the TREX complex which plays a role in mRNA export requires a DEAD box helicase, UAP56 to bind ATP. Here we show that a novel mRNA export co-adaptor, SRAG, binds UAP56 in a mutually exclusive manner with REF, yet both REF and SRAG are found in a fully assembled TREX complex. Interestingly, REF and SRAG stimulate ATP hydrolysis and RNA helicase activity. This implies that UAP56 goes through at least two rounds of ATP hydrolysis to assemble TREX. Within assembled TREX, SRAG functions as an mRNA export co-adaptor and binds synergistically with REF to the TAP mRNA export factor, whose recruitment to TREX triggers UAP56 loss. Depletion of REF or SRAG alone in vivo has a modest effect on mRNA export, but their combined knockdown causes a drastic mRNA export block. Interestingly, the TAP:SRAG interaction is dependent on methylation of SRAG. SRAG binds to TAP in a manner which is mutually exclusive with the TREX component THOC5 and yet TAP, SRAG and THOC5 are found in a single complex in vivo. These data indicate that TREX undergoes substantial rearrangements during its assembly and interaction with TAP, and these rearrangements are driven by UAP56 dependent ATP hydrolysis.