Membrane skeleton protein 4.1 in inner segments of retinal cones

Retinal cone photoreceptors are polarized cells with discrete functional compartments. Little is known of the membrane-skeleton components in cones responsible for the maintenance of cellular morphology and compartmentalization of integral membrane proteins. The authors describe a high density of the membrane-skeleton protein 4.1 in the myoid region of cones in retinas of developing and adult frogs, and adult lizards, turtles, and chicks. In the developing retina, a discrete locus of protein 4.1 is first detected by immunocytochemistry in the cone myoid after the completion of photoreceptor synaptogenesis. Protein 4.1 immunoreactivity expands to line the plasma membrane of the myoid after cones attain their adult proportions. Nonerythroid alpha-spectrin and protein 4.1 colocalize in the cone myoid, and both are in close proximity to long bundles of f-actin that traverse the myoid and extend from the cone outer segment to the external limiting membrane. By analogy to the erythrocyte, where protein 4.1 modulates the interaction of spectrin and actin to regulate cell shape, the authors propose that protein 4.1 interacts with nonerythroid alpha-spectrin and f-actin in cones and plays a role in the maintenance of cone inner segment morphology.