THE DISTRIBUTION OF NA-ATPASE AND 5A11 ANTIGEN IN APICAL MICROVILLI OF THE RETINAL-PIGMENT EPITHELIUM IS UNRELATED TO ALPHA-SPECTRIN(,K+)

The retinal pigment epithelium was used to study the relationship betw een the cortical cytoskeleton and two plasma membrane proteins that as sociate with it. These proteins were the Na+,K+-ATPase, an ion pump, a nd the 5A11 antigen, a member of the immunoglobulin superfamily of rec eptor proteins. The cytoskeleton was marked by two of its constituents , alpha-spectrin and ankyrin. Ankyrin links the Na+,K+-ATPase to spect rin in many cells. The RPE is of interest, because unlike most epithel ia it distributes the Na+,K+-ATPase to the apical membrane. The develo pment of polarity was studied during chick embryogenesis. On embryonic day 6 (E6), each of these proteins was observed in the apical and lat eral plasma membranes. As development proceeded, only the Na+,K+-ATPas e was removed from the lateral membranes. Beginning on E12, ankyrin, s pectrin and 5A11 appeared together in patches along the basal plasma m embrane. By E16, these patches coalesced into a uniform distribution a long the basal membrane. At the apical pole, alpha-spectrin appeared n ear the base of the microvilli, but was undetected in the microvilli t hemselves. This distribution resembled the distribution of alpha-spect rin in the intestine and proximal kidney tubule. By contrast, a pool o f ankyrin and 5A11 and nearly all the Na+,K+-ATPase appeared in the mi crovilli. Despite its segregation from alpha-spectrin, the Na+,K+-ATPa se appeared to associate with a macromolecular complex, as judged by e xtraction with Triton X-100. Changes in spectrin distribution could no t be related to changes in isoform expression, as only one isoform of beta-spectrin was detected by co-immunoprecipitation with alpha-spectr in. By contrast, multiple ankyrin-like peptides could be identified by immunoblotting. These data illustrate some of the unique properties o f RPE microvilli. These properties prevent the Na+,K+-ATPase from comp lexing with the alpha-spectrin-based cytoskeleton by sequestering the enzyme into the compartment where its activity is required.