Large conductance, calcium-activated potassium channels (maxi K- or BK-channels) can be regulated by arachidonic acid produced by c-Phospholipase A(2) (c-PLA(2)). Since in every excised patch from GH3 cells where there was BK-channel activity, treatment with either a stimulator or inhibitor of c-PLA(2) resulted in a corresponding increase or decrease in BK-channel activity, we hypothesized that there must be a close association between BK-channel proteins and c-PLA(2) in the cell membrane. To test this hypothesis, we first determined whether the two proteins would co-immunoprecipitate. We then used confocal imaging of fluorescently tagged proteins to determine where in the cells BK-channel proteins and c-PLA(2) co-localize. The alpha-subunit of the BK-channel was strongly co-immunoprecipitated by c-PLA(2) antibodies, suggesting that most of the BK channel alpha-subunits are associated with c-PLA(2). This interaction was not affected by pharmacologically inhibiting c-PLA(2) suggesting that the association does not require functionally active c-PLA(2). Following dual immunohistochemical labeling and confocal microscopy, image analysis revealed that in the cytosol there was some co-localization, but most of the c-PLA(2) was separate from BK-channel proteins. On the other hand, the c-PLA(2) and BK-channel proteins at the plasma membrane were strongly co-localized. Immunoprecipitation experiments conducted with plasma membrane proteins support these findings. We conclude that c-PLA(2) is likely physically associated with BK-channel proteins at the cell surface. (c) 2006 Elsevier Inc. All rights reserved.