Two RNA-Binding Regions of PCID2, a Subunit of the TREX-2 mRNA Nuclear Export Complex, Competitively Interact with the 3' Noncoding Region of ras2 mRNA.

PCID2 protein is a subunit of the eukaryotic complex TREX-2, which is responsible for nuclear export of mRNA. PCID2 plays an important role in the complex, being responsible for the recognition and binding of the mRNA molecule. PCID2 of D. melanogaster interacts with a region of ras2 (fr4_2) mRNA and has two interaction sites located in its PCI domain: the M-PCID2 region, which non-specifically binds to mRNA, and the C-terminal part (C-PCID2), which specifically recognizes the ras2 fr4_2 mRNA sequence. At the same time, specific binding to C-PCID2 requires a preliminary nonspecific interaction with M-PCID2. It remains unclear how the transition from primary nonspecific interaction to specific interaction occurs: whether both regions interact with RNA simultaneously or whether the nonspecific interaction is required only at the first step for subsequent specific binding. This study showed that the binding of M-PCID2 and C-PCID2 to ras2 fr4_2 RNA is competitive. M-PCID2 binds more efficiently and displaces C-PCID2 from the complex with the ras2 mRNA fragment. Thus, additional factors are required to replace the M-PCID2 contact by C-PCID2 during the interaction of the full-length PCID2 protein with ras2 mRNA. We also showed that point mutations in M-PCID2 that disrupt the interaction of the full-length PCID2 with RNA result in a greater association of M-PCID2 with RNA. It is likely that the increased affinity of M-PCID2 for RNA disrupts the ability to replace M-PCID2 with C-PCID2 within the full-length PCID2.