POLARIZED DISTRIBUTION OF NA,K-ATPASE IN HONEYBEE PHOTORECEPTORS IS MAINTAINED BY INTERACTION WITH GLIAL-CELLS

Arthropod photoreceptors are polarized cells displaying distinct surfa ce domains. The distribution of the Na,K-ATPase (sodium pump) over the se domains was examined in the honeybee photoreceptor using a monoclon al antibody that specifically recognizes the sodium pump alpha-subunit (approximately 100 kDa). We find that the sodium pump is restricted t o sites of the nonreceptive photoreceptor surface closely juxtaposed t o glial cells; no sodium pumps were detected on the glia-free domains of the nonreceptive surface and on the light-sensitive microvillar mem branes. In order to determine the role of photoreceptor-glia contact i n maintaining this polarized pump distribution, we assayed the distrib ution of the Na,K-ATPase after experimentally influencing photorecepto r-glia contact. Sodium pumps were present on the entire nonreceptive p hotoreceptor surface when photoreceptor-glia contact was removed by is olating the photoreceptors. Remodeling photoreceptor-glia contact by i ncubation in hyperosmotic saline caused a redistribution of sodium pum ps on the photoreceptor surface corresponding to the redistribution of glial cells. We show, further, that both photoreceptor-glia contact a nd Na,K-ATPase distribution are independent of extracellular Ca2+. No junctional structures were observed at the borders between Na,K-ATPase -positive and Na,K-ATPase-negative membrane domains. Together, these r esults suggest that adhesion of glial cells to the photoreceptors play s a crucial role in the maintenance of the polarized distribution of N a,K-ATPase in the honeybee photoreceptors. The Ca2+-independent adhesi on of glial cells to the photoreceptor surface may trap the pump molec ules at the sites of photoreceptor-glia contact.