Regulatory domains of erythrocyte ankyrin.

This report provides evidence for regulatory domains of erythrocyte ankyrin that modulate associations of this protein with the anion transporter and spectrin. Two domains have been identified that are located at opposite ends of the polypeptide chain. One domain (Mr = 20,000), which is released by calpain, is primarily involved in regulation of the association of ankyrin with the anion transporter. The Mr = 195,000 fragment remaining after calpain cleavage binds to ankyrin-depleted inside-out vesicles with a 8-fold reaction in affinity, although with a 2-fold increase in number of high affinity sites. Cleavage of ankyrin by calpain induces a reduction in the frictional ratio from 1.55 to 1.33 suggesting either that the calpain-sensitive domain is present as a tail extending from a globular domain, or that upon cleavage ankyrin undergoes a major change in conformation. The other proposed regulatory domain is missing in protein 2.2, a form of ankyrin present in human erythrocytes that has a molecular weight about 29,000 smaller than ankyrin. Protein 2.2 is distinct from the calpain fragment based on peptide maps and reaction with domain-specific antibodies. The activity of the domain deleted from protein 2.2 has been inferred by comparison of ankyrin and protein 2.2, with the assumption that differences between these proteins are due to the missing domain. Protein 2.2 is an activated form of ankyrin that has a 3-fold higher affinity for spectrin and binds to twice the number of high affinity anion transporter sites. These observations suggested that removal of terminal domains of ankyrin may have a physiological role in modulation of ankyrin activity.