Promiscuity in post-transcriptional control of gene expression: Drosophila sex-lethal and its regulatory partnerships

Abstract

The Drosophila RNA-binding protein Sex-lethal (Sxl) is a potent post-transcriptional regulator of gene expression that controls female development. It regulates the expression of key factors involved in sex-specific differences in morphology, behavior, and dosage compensation. Functional Sxl protein is only expressed in female flies, where it binds to U-rich RNA motifs present in its target mRNAs to regulate their fate. Sxl is a very versatile regulator that, by shuttling between the nucleus and the cytoplasm, can regulate almost all aspects of post-transcriptional gene expression including RNA processing, nuclear export, and translation. For these functions, Sxl employs multiple interactions to either antagonize RNA-processing factors or to recruit various coregulators, thus allowing it to establish a female-specific gene expression pattern. Here, we summarize the current knowledge about Sxl function and review recent mechanistic and structural studies that further our understanding of how such a seemingly 'simple' RNA-binding protein can exert this plethora of different functions.

Keywords: RNA-binding protein; alternative splicing; post-transcriptional regulation of gene expression; sex determination; sex-lethal; translational control.

Figure 1

Sxl‐mediated regulation of alternative splicing. (A) Autoregulatory splicing of sxl mRNA. Sxl associates with regulatory sequences that flank the alternative, male‐specific exon E3 (poison exon) to suppress its inclusion in the mature transcript. Regulation not only requires individual proteins of the m⁶A methyltransferase complex and the reader protein YT521‐B (1) but also involves direct contacts of Sxl with components of the splicing machinery (2). (B) Sxl‐dependent regulation of alternative 3′ splice site usage in transformer mRNA. To prevent utilization of the proximal 3′ splice site (proximal 3′ss), Sxl competes with U2AF50 for binding of a U‐rich sequence (red), diverting splicing to the distal, female‐specific splice site (♀3′ss). (C) Sxl‐mediated intron retention in the 5′ UTR of msl‐2 mRNA. By competition for the same RNA‐binding sites, Sxl antagonizes Rox8/TIA and U2AF50 to prevent splicing.

Figure 2

Sxl‐dependent alternative polyadenylation of enhancer of rudimentary mRNA. Activation of the proximal poly(A) site (proximal pA) requires the binding of CstF to a downstream RNA sequence (red). Sxl can compete with and displace CstF‐64, diverting it to a downstream site. This results in activation of a distal polyA‐site (♀ pA) and an extended 3′ UTR in female flies.

Figure 3

Sxl controls translation of its targets by multiple mechanisms. (A) Translational repression of msl‐2 mRNA involves two blocks to translation initiation. Bound to the 3′ UTR regulatory elements, Sxl recruits the corepressor protein UNR (1). Unr interacts directly with the polyA‐binding protein (PABP) to prevent recruitment of ribosomal preinitiation complexes (43S PIC) to the 5′ end of the RNA. Ribosomes escaping this first regulatory mechanism are then challenged by additional molecules of Sxl bound to the 5′ UTR that prevent scanning ribosomes from reaching the msl‐2 open reading frame (2) (modified from [84]). (B) Translational repression of nanos mRNA in the female germline depends on a U‐rich RNA element in the 3′ UTR of the RNA (shown in red). Besides Sxl, the proteins Mei‐P26, Bam, and Bgcn are required for regulation and all four proteins appear to form a complex to repress translation by a yet unknown mechanism.

Figure 4

Structural insights into Sxl function. (A) Sxl bound to the proximal pyrimidine tract of tra mRNA [82; PDB: 1B7F]. The two RRMs of Sxl (gray) specifically recognize the U‐rich pyrimidine tract of tra mRNA (green). RNA contacts are mostly established with the beta‐sheet surfaces of the two RRMs. (B) Structure of the Sxl‐Unr repressor complex bound to a fragment of msl‐2 mRNA [83; PDB: 4QQB]. Binding of Sxl (gray) to the 3′ UTR regulatory elements of msl‐2 mRNA (green) occurs in a manner comparable to recognition of tra mRNA. Upon binding of Unr (cold shock domain 1, shown in orange), additional, ternary contacts are formed that involve the side of RRM1 of Sxl and allow specific recognition of an extended stretch of the RNA.