The cardiac Na+-Ca2+ exchanger binds to the cytoskeletal protein ankyrin

Na+-Ca2+ exchange is the major pathway of Ca2+ efflux during excitation-contraction coupling in cardiac muscle. The Na+-Ca2+ exchanger is present in cardiac transverse tubules with an apparent high density (Frank, J.S., Mottino, G., Reid, D., Molday, R. S., and Philipson, K.D. (1992) J. Cell Biol. 117, 337-345). The mechanism for this localization is unknown but may involve interactions with the cytoskeleton. In the present study, we examined the interaction of the Na+-Ca2+ exchanger with the cytoskeletal protein ankyrin. On immunoblots of isolated canine cardiac sarcolemma, an antibody raised against purified rabbit red blood cell-ankyrin (RBC-ankyrin) recognized a 220-kDa protein, which is the same size as RBC-ankyrin. Alkaline extraction of sarcolemma removed this protein. The Na+-Ca2+ exchange protein, purified from recombinant baculovirus-infected insect cells, bound 125I-labeled-RBC-ankyrin with a KD of 42 +/- 3 nm. 125I-RBC-ankyrin was co-precipitated by antibodies to the Na+-Ca2+ exchanger after preincubation with solubilized cardiac sarcolemma. Myocardial ankyrin could be localized to both surface and T-tubular sarcolemma by immunofluorescence techniques. These results demonstrate that the cardiac Na+-Ca2+ exchanger binds ankyrin with high affinity. This interaction may be important for localizing the Na+-Ca2+ exchanger to specific domains of the sarcolemma.