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 (PubMed:23870127). Binds with a high affinity to glycophorin and with lower affinity to band III protein. Associates with the nuclear mitotic apparatus. Interacts with calmodulin (PubMed:10692436). Interacts with CENPJ (PubMed:11003675). Interacts with DLG1 (PubMed:7937897). Also found to associate with contractile apparatus and tight junctions. Interacts with NUMA1; this interaction is negatively regulated by CDK1 during metaphase and promotes anaphase-specific localization of NUMA1 in symmetrically dividing cells (PubMed:23870127). Interacts with ATP2B1; regulates small intestinal calcium absorption through regulation of membrane expression of ATP2B1 (By similarity). Phosphorylated at multiple sites by different protein kinases and each phosphorylation event selectively modulates the protein's functions. Phosphorylation on Tyr-660 reduces the ability of 4.1 to promote the assembly of the spectrin/actin/4.1 ternary complex. O-glycosylated; contains N-acetylglucosamine side chains in the C-terminal domain. The disease is caused by variants affecting the gene represented in this entry.