Protein 4.1 is a major structural element of the erythrocyte membrane skeleton. It plays a key role in regulating membrane physical properties of mechanical stability and deformability by stabilizing spectrin-actin interaction. May be required for dynein-dynactin complex and NUMA1 recruitment at the mitotic cell cortex during anaphase. Binds with a high affinity to glycophorin and with lower affinity to band III protein. Associates with the nuclear mitotic apparatus. Binds calmodulin. Found exclusively in photoreceptors following the terminal mitosis of retinal neurons. When retinal synaptogenesis is complete, protein 4.1 is also expressed in the inner retina. In adult amphibian retinas, protein 4.1 is detected in photoreceptors, bipolar cells, and ganglion cell axons. Phosphorylated at multiple sites by different protein kinases and each phosphorylation event selectively modulates the protein's functions.