The balance between isoforms of the Prickle LIM domain protein is criticalfor planar polarity in Drosophila imaginal discs

The tissue polarity mutants in Drosophila include a set of conserved gene p roducts that appear to be involved in the control of cytoskeletal architect ure. Here we show that the tissue polarity gene prickle (pk) encodes a prot ein with a triple LIM domain and a novel domain that is present in human, m urine, and Caenorhabditis elegans homologs which we designate PET. Three tr anscripts have been identified, pk, pkM, and sple, encoding 93-, 100-, and 129-kD conceptual proteins, respectively. The three transcripts span 70 kb and share 6 exons that contain the conserved domains. The pk and sple trans cripts are expressed with similar tissue-specific patterns but have qualita tively different activities. The phenotypes of pk mutants, and transgenic f lies in which the different isoforms are overexpressed show that the balanc e between pk and Sple is critical for the specification of planar polarity. In addition, these phenotypes suggest a tessellation model in which the al ignment of wing hairs is dependent on cell shape and need not reflect fine- grained positional information. Lack of both pk and sple transcripts gives a phenotype affecting the whole body surface that is similar to those of di shevelled and frizzled (fz) suggesting a functional relationship between pk and fz signaling.