ROLES FOR RAC1 AND CDC42 IN PLANAR POLARIZATION AND HAIR OUTGROWTH INTHE WING OF DROSOPHILA

The wing of Drosophila melanogaster is covered by an array of distally pointing hairs. PI hair begins as a single membrane outgrowth from ea ch wing epithelial cell, and its distal orientation is determined by t he restriction of outgrowth to a single distal site on the cell circum ference (Wong, L., and P. Adler. 1993. J. Cell Biol. 123:209-211.). We have examined the roles of Cdc42 and Rad in the formation of wing hai rs. We find that Cdc42 is required for localized actin polymerization in the extending hair. Interfering with Cdc42 activity by expression o f a dominant negative protein abolishes both localized actin polymeriz ation and hair outgrowth. In contrast, Rad is important for restrictin g the site at which hairs grow out. Cells expressing the dominant nega tive Rac1N17 fail to restrict outgrowth to a single site and give rise to multiple wing hairs. This polarity defect is associated with distu rbances in the organization of junctional actin and also with disrupti on of an intricate microtubule network that is intimately associated w ith the junctional region. We also find that apical junctions and micr otubules are involved in structural aspects of hair outgrowth. During hair formation, the apical microtubules that point distally elongate a nd fill the emerging wing hair. As the hair elongates, junctional prot eins are reorganized on the proximal and distal edges of each cell.