In eukaryotic cells, RNAs are synthesized in the nucleus and then transported into the cytoplasm (to function, for instance, as enzymes or to be translated into polypeptides). Many RNAs are modified after synthesis and before transport in processes such as polyadenylation, editing, and splicing; how the signals indicating that processing is complete are coordinated with those mediating transport is not yet understood.
Earlier work has shown that the simian type D retrovirus encodes a constitutive transport element (CTE) that serves to circumvent the customary checks and enables the unspliced genomic RNA to slip through the transport pathway into the cytoplasm, a requirement for proper retroviral replication. Liker et al. now describe the crystal structure of the CTE-binding fragment of human TAP (the homolog of yeast messenger RNA export factor Mex67p). They observe two domains: one is a ribonucleoprotein or RNP domain, and the other is a leucine-rich repeat or LRR domain. Of particular interest is the structural similarity of these with the heterodimer of the RNA splicing factors U2B'' and U2A'. Taken together with mutagenesis results, this is highly suggestive of a common mode of RNA recognition in which the LRR acts on the RNP domain to promote RNA binding, to incompletely spliced cellular mRNA in the case of U2B''-U2A', and to the CTE element of retroviral genomic RNA in the case of TAP. -- GJC
EMBO J. 19, 5587 (2000).
