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

Photoreceptor and accessory cells in the insect compound eye exhibit a char acteristic architecture, probably established and maintained by the contrib ution of membrane-associated cytoskeletal elements. The present study ident ifies and localizes nonmuscle myosin-II in honeybee photoreceptors by use o f an affinity-purified antibody against scallop muscle myosin-II heavy chai n (MHC). Western blot analysis and immunofluorescence staining confirmed cr oss-reactivity of the antibody with honeybee muscle MHC. In the compound ey e, the antibody identified a protein that comigrated with muscle MHC on sod ium dodecylsulfate-polyacrylamide gels. Association with the cytoskeleton, ATP-dependent binding to exogenous actin filaments, and cross-reactivity wi th several other antibodies against MHC, including an antibody to Drosophil a nonmuscle MHC, support the conclusion that the cross-reacting protein rep resents nonmuscle MHC. Confocal immunofluorescence microscopy on honeybee e yes showed that the motor protein was highly enriched at distinct regions o f the photoreceptor surface next to the Light-receptive compartment, the rh abdom. To determine the function of myosin-II in these cells, retinal tissu e was incubated with 2,3-butanedione 2-monoxime (BDM), an inhibitor of myos in 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 af ter exposure to BDM. These results suggest that the activity of nonmuscle m yosin-II in the visual cells exerts tension on a distinct surface region ne xt to the rhabdom, contributes to the positioning of the rhabdom, and, thus , plays a role in maintaining the cellular architecture within the compound eye. (C) 2001 Wiley-Liss, Inc.