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. 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, CENPJ and DLG1. Also found to associate with contractile apparatus and tight junctions. Interacts with NUMA1; this interaction is negatively regulated by CDK1 during metaphase and promotes for anaphase-specific localization of NUMA1 in symmetrically dividing cells. Interacts with ATP2B1; regulates small intestinal calcium absorption through regulation of membrane expression of ATP2B1 (By similarity). O-glycosylated; contains N-acetylglucosamine side chains in the C-terminal domain. Phosphorylated at multiple sites by different protein kinases and each phosphorylation event selectively modulates the protein's functions.