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. Recruits DLG1 to membranes. 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 and DLG1. A number of isoforms are produced. Phosphorylated at multiple sites by different protein kinases and each phosphorylation event selectively modulates the protein's functions.