Abstract. Adducin is a membrane-skeletal protein which is a candidate to promote assembly of a spectrin-actin network in erythrocytes and at sites of cellcell contact in epithelial tissues . The complete sequence of both subunits of human adducin, alpha (737 amino acids), and beta (726 amino acids) has been deduced by analysis of the cDNAs . The two subunits have strikingly conserved amino acid sequences with 49% identity and 66% similarity, suggesting evolution by gene duplication . Each adducin subunit has three distinct domains: a 39-kD NH2-terminal globular protease-resistant domain, connected by a 9-kD domain to a 33-kD COOH-terminal protease-sensitive tail comprised almost entirely of hydrophilic amino acids . The tail is responsible for the high frictional ratio of adducin noted previously, and was visualized by EM . The head domains of both adducin subunits exhibit a limited sequence similarity with the NH2-terminal ac-LASMA membranes of eukaryotic cells contain a system of structural proteins known as the spectrin-based membrane skeleton that has a general role in organi zation ofcertain integral membrane proteins and of coupling these proteins to cytoplasmic proteins (2,16,29) . Spectrin, the principal component of the membrane skeleton, is a rodshaped protein that associates with F-actin ateach end . Spectrin and actin together with accessory proteins form a regular geodesic domelike structure in erythrocytes where short actin filaments are associated with five and seven spectrin molecules to form a polygonal network (5, 28) . The striking images of the spectrin skeleton raise the issue of how such a structure could be assembled and has focused attention on the accessory proteins that interact with spectrin and actin at spectrin-actin junctions (1) .Adducin is a membrane-skeletal protein that promotes association of spectrin with actin, and this interaction is regulated by calcium/calmodulin (14, 31) . Adducin is comprised of two subunits with M 103,000 (alpha) and 97,000 (beta) and was originally purified from human erythrocytes based on its calmodulin-binding activity (13) . Adducin also is a substrate for protein kinase C in intact cells as well as under in vitro conditions (6,27,33,39). A protein closely related

. Ankyrins are a family of membraneassociated proteins that can be divided into two immunologically distinct groups : (a) ery . throcyte-related isoforms (ankyrinR) that have polarized distributions in particular cell types; and (b) brain-related isoforms (ankyrinB) that display a broader distribution . In this paper, we report the isolation and sequences of cDNAs related to two ankyrinB isoforms, human brain ankyrin 1 and 2, and show that these isoforms are produced from alternatively spliced n1RNAs of a single gene . Human brain ankyrin 1 and 2 share a common NH2-terminus that is similar to human erythrocyte ankyrins, with the most striking conservation occurring between areas composed of a repeated 33-amino acid motif and between areas corresponding to the central portion of the spectrin-binding domain . In contrast, COOH-terminal sequences of brain ankyrin 1 and 2 are distinct from one another and from human erythrocyte ankyrins, and thus are candidates to mediate protein interactions that distinguish these isoforms . The brain ankyrin 2 cDNA sequence includes a stop codon and encodes a polypeptide with a predicted mo-MNY integral membrane proteins exhibit restricted lateral mobilities and are confined to specific areas of the plasma membrane (7). As a result, the sur faces of most eukaryotic cells are highly organized and often participate in diverse functions such as formation of cell-cell junctions, localized flux of ions, and polarized secretion of intracellular vesicles . The ordered distribution of these proteins cannot be maintained by the fluid phospholipid bilayer alone, but requires that at least some of these proteins be linked to underlying cytoskeletal supports. One of the beststudied examples of such an association involves ankyrin, a protein that links the anion exchanger of erythrocytes to a meshwork of defined proteins known as the spectrin-based membrane skeleton (reviewed in 3 and 4).Many ofthe proteins associated with the erythrocyte membrane skeleton have been found in cells other than erythrocytes, suggesting a role for this structure in the organization Manabu Kunimotds present address is The