The study of integrins in cell culture systems has offered many insights and assays for integrin function. In this way, we can test whether genetic removal of a protein produces defects consistent with its proposed role. We are complementing this approach with a reverse genetic approach consisting of the identification of mutations in genes encoding Drosophila orthologs of vertebrate proteins implicated in integrin function. The wing blister phenotype has been used successfully in genetic screens to identify other genes required for this integrin-mediated adhesion in the adult wing ( Prout et al. In large clones, mutations in the αPS2 subunit only cause a wing blister when present on the ventral side, while mutations in the αPS1 subunit only cause blisters on the dorsal side, consistent with their enriched expression on the different sides ( Brower et al. This adhesion involves two different integrins, containing two of the five α subunits present in the Drosophila genome ( Adams et al. When clones of cells homozygous for integrin mutations are produced in the wing, the mutant cells do not adhere to the opposing cell layer, resulting in a wing blister. 1990 Brabant and Brower 1993 Brower et al. Similarly, the PS integrins are required to hold together the two epithelial cell layers that form the adult wing ( Brower and Jaffe 1989 Wilcox et al. This demonstrates that the integrins are necessary to link the cell to the ECM, but are not the only molecule mediating interactions between the plasma membrane and the cytoskeleton. In the detached muscles, the plasma membrane is separated from the ECM, but the muscle actin filaments remain linked to the plasma membrane ( Newman and Wright 1981 Prokop et al. In the absence of PS integrins, the muscle attachment to the epidermis via the ECM fails, so the muscles detach and round up ( Wright 1960). Several characteristic PS integrin mutant phenotypes are caused by loss of adhesion between adjacent cell layers. In Drosophila, null mutations of the genes encoding the position-specific (PS) integrins cause diverse morphogenetic defects during development and embryonic lethality (reviewed in Brown et al. Through these interactions, integrins connect the extracellular matrix to the cytoskeleton, providing strong cell adhesion to the ECM ( Hynes 1992 Cheresh and Mecham 1994). The short cytoplasmic domain of the β integrin subunit anchors the cytoskeleton to the plasma membrane via intermediary adaptor proteins. The large extracellular part of both α and β subunits binds proteins within the ECM. Integrins are a large family of transmembrane heterodimeric proteins that mediate such interactions. The interaction of cells with the surrounding extracellular matrix (ECM) affects many aspects of cell behavior, including the migratory properties of cells, their growth, and differentiation. Our data suggest that ILK is a component of the structure linking the cytoskeleton and the plasma membrane at sites of integrin-mediated adhesion. The muscle detachment in ILK mutants is associated with detachment of the actin filaments from the muscle ends, unlike integrin mutants, in which the primary defect is detachment of the plasma membrane from the extracellular matrix. Surprisingly, mutations in the kinase domain shown to inactivate the kinase activity of human ILK do not show any phenotype in Drosophila, suggesting a kinase-independent function for ILK. Consistent with this, an ILK–green fluorescent protein fusion protein colocalizes with the position-specific integrins at sites of integrin function: muscle attachment sites and the basal junctions of the wing epithelium. ILK mutations cause embryonic lethality and defects in muscle attachment, and clones of cells lacking ILK in the adult wing fail to adhere, forming wing blisters. Here we show that the absence of ILK function in Drosophila causes defects similar to loss of integrin adhesion, but not similar to loss of these signaling pathways. Integrin-linked kinase (ILK) was identified by its interaction with the cytoplasmic tail of human β1 integrin and previous data suggest that ILK is a component of diverse signaling pathways, including integrin, Wnt, and protein kinase B.
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