Distribution of nonmuscle myosin-II in honeybee photoreceptors and its possible role in maintaining compound eye architecture.

Abstract

Photoreceptor and accessory cells in the insect compound eye exhibit a characteristic architecture, probably established and maintained by the contribution of membrane-associated cytoskeletal elements. The present study identifies and localizes nonmuscle myosin-II in honeybee photoreceptors by use of an affinity-purified antibody against scallop muscle myosin-II heavy chain (MHC). Western blot analysis and immunofluorescence staining confirmed cross-reactivity of the antibody with honeybee muscle MHC. In the compound eye, the antibody identified a protein that comigrated with muscle MHC on sodium dodecylsulfate-polyacrylamide gels. Association with the cytoskeleton, ATP-dependent binding to exogenous actin filaments, and cross-reactivity with several other antibodies against MHC, including an antibody to Drosophila nonmuscle MHC, support the conclusion that the cross-reacting protein represents nonmuscle MHC. Confocal immunofluorescence microscopy on honeybee eyes showed that the motor protein was highly enriched at distinct regions of the photoreceptor surface next to the light-receptive compartment, the rhabdom. To determine the function of myosin-II in these cells, retinal tissue was incubated with 2,3-butanedione 2-monoxime (BDM), an inhibitor of myosin activity. BDM treatment resulted in an increase in surface curvature at precisely those membrane areas that exhibited intense immunoreactivity for MHC. Moreover, the positioning and alignment of the rhabdoms was altered after exposure to BDM. These results suggest that the activity of nonmuscle myosin-II in the visual cells exerts tension on a distinct surface region next to the rhabdom, contributes to the positioning of the rhabdom, and, thus, plays a role in maintaining the cellular architecture within the compound eye.