BIOLOGICAL OVERVIEW

Corkscrew, a component of the Ras signaling pathway, is a protein tyrosine phosphatase (PTP). PTPs, it should be kept in mind, remove (or lyse) phosphates, as the name phosphatase implies. csw was initially identified as a maternal effect mutation of the terminal system: CSW functions in the same signaling pathway as the receptor tyrosine kinase (RTK) Torso (Perkins, 1992).

In a signaling pathway, RTKs signal for action -- the binding of phosphates: RTKs are said to have positive or upward regulatory effects. PTPs are thought to function in a complementary, albeit antagonistic manner to RTKs, signaling for quiescence, or the removal of phosphates: they are said to have negative, or downward regulatory effects. Subsequent studies of CSW reveal a paradox: CSW appears to initiate positive activity (that is, it aids in sending a signal for phosphate binding) in the Ras pathway phosphorylation cascade. This apparent paradox offers insight into the functioning of the early stages of Ras pathway signaling.

The phosphatase CSW, unlike the RTK Torso, is required during zygotic development. Two factors contribute to the belief that CSW has zygotic roles. First, zygotically acting csw mutations, unlike tor, result in embryonic lethality. Secondly, unlike tor, the csw maternal effect phenotype is partially paternally rescuable. For example, the posterior midgut invagination, and hence the midgut, is entirely deleted in null embryos, whereas in paternally rescued embryos the midgut is malformed and reduced in size. CSW also functions in the development of ventral cell fate in the embryo. Significantly fewer cells stain with Fas III in csw mutant embryos, relative to wild type. This apparent loss of ventral cell fate is accompanied by a concomitant expansion of lateral cell fates. For example, the laterally positioned cells of the tracheal pits are closer to the midline in csw mutants. Likewise, the ventrolaterally positioned Keilin's organs appear closer together than normal in csw mutants (Perkins, 1996).

CSW also plays a role in embryonic CNS development. In null csw embryos, which display a more severe phenotype, the horizontal commissures are collapsed. This is in contrast to paternally rescued csw embryos that display the less severe phenotype: the commissures are separated along the ventral midline. In both paternally rescued and null mutant embryos the longitudinal axon tracts are rudimentary and discontinuous. Similar phenotypes have been reported in embryos mutant for EGF Receptor (Perkins, 1996).

To determine whether CSW is involved in signaling from the Breathless receptor tyrosine kinase during tracheal development, phenotypes of mutant csw embryos were examined using a tracheal-specific marker. In mutant embryos, tracheal cell precursors are produced normally, but their subsequet migration, which generates the tracheal tree, appears defective. An incomplete and disconnected system of tracheal branches is the final result. Thus it appears that CSW operates positively in BTL signaling for the formation of the mature larval tracheal network (Perkins, 1996).

CSW also plays a role in the development of adult structures. Mutant csw adults reveal consistent defects including absence of one or both of the distal-most antennal segments, the aristae; lack of one or more of the distal-most leg segments, the tarsal claws; incomplete formation of distal portions of wing vein L5 (and often loss of L4), and eyes with disorganized, reduced in numbers ommatidia and ommatidial bristles. The phenotypes of csw mutant adults are similar to those reported for viable EGF-R mutations, suggesting that csw functions positively during imaginal development in the EGF-R signaling pathway (Perkins, 1996).

CSW is also required during oogeneis for normal follicle cell development. Follicle cell development is known to be stimulated by an asymmetrically localized signal from Gurken, originating in the oocyte. Phenotypes of eggs derived from partially zygotically rescued csw females were examined to determine whether CSW plays any role in EGF-R signaling in dorsal follicle cells. Such females lay eggs with fused dorsal appendages that correspond to an expansion of ventral chorionic cell fates at the expense of dorsal chorionic cell fates. This effect is similar to the role of EGF-R during oogenesis and suggests that CSW operates downstram of EGF-R in the establishment of dorsal follicle cell fates (Perkins, 1996).

RTK signaling still operates in the absence of CSW activity, as evidenced by the incomplete phenotype of genetically null csw alleles compared to torso mutants. In addition, the role of CSW is positive, expediting the RTK signals in response to ligand binding. What then is the role of CSW in RTK signaling? It may help to think of CSW as a docking protein. The presence of two SH2 domains suggests that they could directly bind activated receptor tyrosine kinases. This model is supported by studies with the mammalian phosphatase SHP-2, which physically associates with PDGF and EGF receptors and the insulin receptor substrate IRS-1. Further, upon binding to the EGF or PDGF receptors, SHP-2 becomes tyrosine phosphorylated. One of the sites of tyrosine phosphorylation within SHP-2 provides a binding site for Grb2. These findings suggest a mechanism whereby upon PDGF receptor activation, SHP-2 is recruited to the receptor and then becomes tyrosine phosphorylated, which in turn recruits the GRb2/Sos complex to the membrane, thereby activating Ras. The binding of SHP-2 to PDGF and EGF receptors is analagous to the binding of CSW to EGF-R in Drosophila. In both cases the phosphatase helps recruit additional proteins involved in the signaling process. However, this model still does not account for the PTPase catalytic activity of CSW. It is likely that the phosphatase activity acts only after the assembly of the whole signaling complex, and then the phosphatase activity tones down the strength of the signal, the conventional role ascribed to phosphatases (Perkins, 1996 and references).

GENE STRUCTURE

Two transcripts, each one longer than the prevalent 4.7 kb form (6.0 and 7.2 kb), are first observed during late embryogenesis, but upon longer exposure are also observed throughout larval, pupal and adult stages (Perkins, 1992).

Bases in 5' UTR - 140

Exons - 2

Bases in 3' UTR - 1866

PROTEIN STRUCTURE

Amino Acids - 841

Structural Domains

The csw gene encodes a putative nonreceptor protein tyrosine phosphatase covalently linked to two N-terminal SH2 domains, which is similar to the mammalian PTP1C protein (Perkins, 1992).

The CSW PTPase domain is similar to that of PTP1C, with which it shares 58% identity. The CSW PTPase domain is unusual among the known PTPase proteins, since it is interrupted by a hydrophilic serine- and cysteine-rich stretch of approximately 150 aa, the PTPase insert, which shares no homologies with other PTPase proteins. The N-terminal SH2 domains of the CSW protein share between 45% and 65% identity with the next most closely related SH2 domains, those from the mammalian phosphatase PTP1C (Perkins, 1992).

date revised: 17 Dec 96 

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