High impact information on grk

• The localized ligand Gurken activates Egfr to different levels in different follicle cells [1].
• The TGF alpha-like ligand Gurken signals from the oocyte to the receptor in the overlying somatic follicle cells [2].
• During oogenesis, the grk transcript becomes asymmetrically localized to the dorsal corner of the oocyte [3].
• Analysis of germ-line mosaics demonstrates that both ovarian and embryonic phenotypes will be produced when either the gurken+ gene is removed from the germ line or torpedo+ is removed from the soma [4].
• The germ line therefore regulates the development of the follicle cells through two complementary signaling pathways: Gurken signals twice to control spatial patterning, whereas Delta signals twice to exert temporal control [5].

Biological context of grk

• Hrb27C, Sqd and Otu cooperatively regulate gurken RNA localization and mediate nurse cell chromosome dispersion in Drosophila oogenesis [6].
• Here we show that post-transcriptional regulation of Grk protein levels is required for correct pattern formation. encore (enc), a gene that functions in the regulation of germline mitosis and maintenance of oocyte identity, is also required for the accumulation of Grk protein during oogenesis [7].
• Consistent with such embryonic phenotypes, orb is required for the asymmetric distribution of oskar and gurken mRNAs within the oocyte during the later stages of oogenesis [8].
• Experiments using chimeric eggs composed of a D. virilis oocyte and D. melanogaster follicle cells showed the evolution of DA number was not attributable to germline Gurken (Grk) signaling, but to divergence in events downstream of Grk signaling affecting the rho enhancer activity in somatic follicle cells [9].
• Beside their roles in RNA transport, both motors are involved in nuclear positioning and in exocytosis of Gurken protein [10].

Anatomical context of grk

• This is the first evidence that grk, in addition to its involvement in the genesis of anterior-posterior and dorsal-ventral polarity, is also required for Egfr-dependent development of the follicular epithelium that surrounds each nurse cell/oocyte cluster to form an egg chamber [11].
• Mislocalization of grk and osk transcripts, however, suggests a defect in the microtubule reorganization that occurs during the middle stages of oogenesis and determines axial polarity [12].
• In addition, Grk protein accumulates in large, actin-caged vesicles near the endoplasmic reticulum of stages 6-10 egg chambers [12].
• Expression of fringe is down regulated by Gurken/Epidermal Growth Factor Receptor signalling and is required for the morphogenesis of ovarian follicle cells [13].
• Here, Gurken signals again to specify dorsal follicle cells, which give rise to dorsal chorion structures including the dorsal appendages [14].

Associations of grk with chemical compounds

• We propose that Merlin acts in response to the Gurken signal by apically targeting the signal that initiates axis specification in the oocyte [15].
• The gurken protein has homology to secreted growth factors and may bind to the torpedo/DER receptor tyrosine kinase present on the adjacent follicle cells [16].

Physical interactions of grk

• During Drosophila oogenesis, Cni is required for transport of the TGFalpha growth factor Gurken (Grk) to the oocyte surface [17].
• Finally, we provide evidence that grk mRNA localization and translation are coupled by an interaction between Sqd and the translational repressor protein Bruno [18].
• Using the yeast two-hybrid system we show here, for the first time, that Gurken protein directly binds to the extracellular domain of Egfr [19].
• The function of spoonbill appears to be required for dpp transcription in a specialized population of follicle cells and for the selective transport of grk mRNA from the nurse cells into the oocyte, as well as for its proper localization and translation [20].

Co-localisations of grk

• Cloning of the enc locus and generation of anti-Enc antibodies reveal that enc encodes a large novel protein that accumulates within the oocyte cytoplasm and colocalizes with grk mRNA [21].

Regulatory relationships of grk

• Our data demonstrate a novel role for Hrb27C in promoting grk localization [6].
• We present evidence that enc regulates Grk post-transcriptionally to ensure adequate levels of signaling for establishment of the anterior-posterior and dorsal-ventral axes [7].
• Grk is expressed in the oocyte and activates the Egfr in the surrounding follicle cells during oogenesis [22].
• Bruno regulates gurken during Drosophila oogenesis [23].
• At the stage when dorso-ventral polarity of the follicle cells is established, Sprouty limits the ventral expansion of the activating Gurken signal [24].
• Our results show that Gurken travels towards the lateral/posterior of the egg chamber in the absence of Cbl, suggesting that Cbl actively regulates Gurken distribution through promoting endocytosis and subsequent degradation [25].

Other interactions of grk

• Our phenotypic data argue for a role of lic in a post-transcriptional regulation of the grk gene [26].
• Consequently, in the absence of Cni function, Grk fails to leave the oocyte ER [17].
• In Drosophila oogenesis, the hnRNP Squid (Sqd) functions in the localization and translational regulation of gurken (grk) mRNA [6].
• Brainiac encodes a novel, putative secreted protein that cooperates with Grk TGF alpha in the genesis of the follicular epithelium [11].
• Rab11 and alpha-adaptin localization occurs in the absence of a polarized microtubule cytoskeleton, i.e. in grk null mutants, but is later reinforced and/or refined by Osk, the localization of which is microtubule dependent [27].

Analytical, diagnostic and therapeutic context of grk

• By immunoblotting we detect one major form of the Gurken protein, which likely corresponds to the unprocessed protein [28].

References