Fig. 3. Key factors involved in bulk mRNA export in metazoan cells. Recruitment of UAP56 to mRNA molecules, either by splicing or possibly via one of the other indicated mechanisms, likely represents the key initial step in inducing nuclear mRNA export. UAP56 may then recruit Aly, which in turn binds the Tap-Nxt nuclear RNA export factor. The MPMV CTE RNA binds the Tap-Nxt heterodimer directly, thus obviating the need for upstream factors. UAP56, Tap and Nxt are all essential for bulk mRNA nuclear export, but Aly is not, thus implying that a second, unknown factor may also mediate recruitment of the Tap-Nxt heterodimer by mRNA-bound UAP56 molecules.