BIOLOGICAL OVERVIEW

Localization of Bicoid (BCD) mRNA to the anterior and Oskar (OSK) mRNA to the posterior of the Drosophila oocyte is critical for embryonic patterning. exuperantia (exu) is implicated in BCD mRNA localization, but its role in this process is not understood. Various studies have shown that localized messages are organized into particles, suggesting that a large protein complex may be involved in recognizing, transporting, and anchoring localized messages. Exu is part of a large RNase-sensitive complex that contains at least seven other proteins. One of these proteins is the cold shock domain RNA-binding protein Ypsilon Schachtel (Yps), which binds directly to Exu and colocalizes with Exu in both the oocyte and nurse cells of the Drosophila egg chamber. The Exu–Yps complex also contains OSK mRNA. exu-null mutants are defective in OSK mRNA localization in both nurse cells and the oocyte. It is proposed that Exu is a core component of a large protein complex involved in localizing mRNAs both within nurse cells and the developing oocyte (Wilhelm, 2000 and references therein).

Genetic screens have identified several mutants that have patterning defects due to the mislocalization of BCD and/or OSK mRNAs. Mutations in some genes, such as swallow and staufen, cause only partial disruption of BCD mRNA localization late in oogenesis. However, in exuperantia mutants, defects in BCD mRNA localization occur early in oogenesis and result in BCD mRNA being uniformly distributed in the mature oocyte (Berleth, 1988; St Johnston, 1989). Time-lapse confocal microscopy has shown that green fluorescent protein (GFP)-Exu forms particles that move in a microtubule-dependent manner and accumulate at the anterior and posterior of the oocyte (Theurkauf, 1998). Immunoelectron microscopy has also revealed that Exu is a component of large electron-dense structures called sponge bodies (Wilsch-Brauninger, 1997; Wilhelm, 2000 and references therein).

The pathways by which anterior- and posterior-localized mRNAs arrive at their destinations are poorly understood, although it is generally believed that these RNAs are recognized by different proteins and utilize distinct transport machineries. However, it is proposed that anterior- and posterior-localized mRNAs begin their localization process in the nurse cells using a similar complex, with Exu serving as a common core component. In this model, one of Exu's functions is as a component of an mRNA transport complex, since GFP-Exu particles have been observed to move in a microtubule-dependent manner (Theurkauf, 1998). Consistent with this idea, both OSK and BCD mRNA accumulate in apical patches within nurse cells, and exu mutants disrupt this localization pattern for both mRNAs (St Johnston, 1989; Pokrywka, 1995). It is also proposed that the Exu complex transports mRNAs from the nurse cells to the oocyte as well as within the oocyte, although these transport steps also can be achieved through other redundant mechanisms, such as nurse cell dumping and cytoplasmic streaming. Although the above model places Exu as part of a transport complex, it should be noted that Exu might contribute to the establishment of anchoring once mRNAs reach their final destination (Wilhelm, 2000).

After arriving in the oocyte, BCD- and OSK-containing RNPs must be sorted so that BCD becomes anchored at the anterior, whereas OSK is transported to the posterior pole. Since Yps, Exu, BCD mRNA, and OSK mRNA all first colocalize at the anterior, it is proposed that this sorting decision occurs at the anterior of the oocyte. Evidence for this anterior sorting model comes from genetic studies of staufen (stau) and tropomyosin II (TmII) that show that these proteins do not interfere with anterior localization but rather block the release and transport of OSK transcripts to the posterior. The molecular basis for this sorting decision is unclear, but may involve modifications to the transport machinery or the recruitment of additional factors (Wilhelm, 2000 and references therein).

Whereas Exu and Yps associate with one another independently of mRNA, another component of the ribonucleoprotein complex, the DEAD-box protein, Me31B, associates with Exu and Yps in a RNase-sensitive manner. Me31B is dispensable for the transport of the associated mRNA and proteins molecules to oocytes. Exu, OSK and BicaudalD mRNAs can be transported to the oocyte even in the absence of Me31B. Nevertheless, Me31B is essential for the translational silencing of OSK and BicaudalD mRNAs during their transport to the oocyte. This suggests that Me31B and the Exu-Yps complex bind different regions of the same RNA molecule. These data lead to the speculation that the assembly of a cytoplasmic RNP complex is achieved by binding of functionally different proteins to discrete regions of an oocyte-localizing RNA (Nakamura, 2001).

GENE STRUCTURE

cDNA clone length - 2.1 kb in the female and 2.9 kb in the male

Bases in 5' UTR - 795 and 1543

Exons - 4 and 5

Bases in 3' UTR - 1442 and 1595

PROTEIN STRUCTURE

Amino Acids - 532

Structural Domains and Evolutionary Homologs

How highly conserved are the mechanisms of mRNA localization, a process crucial to Drosophila body patterning? Two components are involved in that process: the exuperantia gene, required for an early step in localization, and the cis-acting signal that directs BCD mRNA localization. The cloned D. melanogaster exu gene has been used to identify the exu genes from D. virilis and D. pseudoobscura. Surprisingly, D. pseudoobscura has two closely related exu genes, while D. melanogaster and D. virilis have only one each. When expressed in D. melanogaster ovaries, the D. virilis exu gene and one of the D. pseudoobscura exu genes can substitute for the endogenous exu gene in supporting localization of BCD mRNA, demonstrating that function is conserved (Luk, 1994).

date revised: 30 September 2001

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