Expression of the beta subunit of spectrin in nonerythroid cells.

Abstract

Antibodies raised against electrophoretically purified chicken erythrocyte fi subunit of spectrin, called "1-spectrin," have been used to demonstrate the presence of an immunoreactive form of this polypeptide in nonerythroid tissues. Immunoautoradiography shows that, in chicken erythrocytes, this antiserum reacts with P-spectrin (Mr 220,000) and another polypeptide (Mr 230,000) that, by two-dimensional tryptic peptide analysis, shows extensive homology with P-spectrin but not with the a subunit of spectrin, called "e-spectrin." Immunoautoradiography and immunoprecipitation of various chicken tissues with this antiserum shows that either one variant or both variants of f-spectrin are expressed. Indirect immunofluorescence reveals that the antiserum reacts with a plasma membrane-associated component of erythroid and some nonerythroid cells. Particularly strong fluorescence is observed in skeletal and cardiac muscle cells where (3-spectrin appears to form a grid-like network along the inner surface ofthe sarcolemma. The noncoordinated distribution of a-and (-spectrin variants indicates that their expression may be tailored to the functional requirements of the plasma membrane in different cells.The fluid mosaic model for membranes, which was developed from early studies on the erythrocyte membrane (1, 2), has been extended to include the notion that the lateral mobility of proteins in the plane ofthe membrane is restricted by interactions between these proteins and the cytoskeleton (3, 4). The nature of these interactions remains, to a great extent, an unresolved problem despite the fact that several membrane-associated proteins have been implicated in this process. In mammalian erythrocytes, actin is attached to the plasma membrane through a high molecular weight protein, spectrin, which is found on the inner surface of the plasma membrane: it is composed of two nonidentical subunit polypeptides herein termed "a-spectrin" (Mr 240,000) and "/-spectrin" (Mr 220,000) (for reviews, see refs. 5-7). Spectrin forms a crosslinked network with actin filaments (8,9) and is linked to the membrane through the association of the 8 subunit with ankyrin (10, 11). Ankyrin associates with the membrane by binding to a subset of the transmembrane anion transporters (5, 7). Thus, in the erythrocyte, spectrin appears to play a pivotal role in linking the plasma membrane to the cytoskeleton and in restricting the mobility of certain transmembrane polypeptides. In the past, the failure to detect immunoreactive forms of spectrin in nonerythroid tissues (12, 13) has limited generalizations regarding the role of spectrin in the highly specialized erythrocyte. Recently, however, several laboratories have identified a spectrinlike protein in nonerythroid cells (14)(15)(16)(17)(18)(19)(20); the protein shows several biochemical properties common with a-spectrin from erythrocytes (18)(19)(20) (24), and their proteins were separated by NaDodSO4/10% or 12.5% polyacrylamide gel electrophoresis based on the system of Laemmli (25) as modi...

Avian lens spectrin: subunit composition compared with erythrocyte and brain spectrin.

“…Minor amounts of polypeptides antigenically related to erythrocyte ~-spectrin are also detected. The fact that the lens is avascular and that only decapsulated lenses free of muscle and connective tissue were used in this and our previous study (29), demonstrates unequivocally that lens cells specifically express these ~-spectrin-related polypeptides.…”

“…The two-dimensional peptide maps of the -r-spectrin from lens and brain show that there is considerable homology between the polypeptides, although there are a few peptides which appear to be specific to each. Chicken erythrocytes express two immunologically related polypetides (/~-and ~'-spectrin) which have very similar, but not identical, peptide maps (29). Neither lens nor brain V-spectrin show any appreciable homology with erythroid B-or/~'-spectrin.…”

“…3, reference 37). Similarly, the lower molecular weight polypeptides below erythrocyte ~-spectrin (lane 5) also represent proteolytic fragments (see reference 29). The presence of two/~-spectrin variants in chicken erythrocytes is not clear in lane 5 due to the long exposure of the autoradiogram required to detect cross-reacting polypeptides in lens.…”

“…Chicken erythrocyte membranes were prepared as described (15). Antibodies against electrophoretically purified chicken erythrocyte a-Spectrin and B-spectrin have been described (29,37). The igG fractions were partially purified by precipitation with ammonium sulfate at 50% saturation at 4"C.…”

“…In the erythrocyte, spectrin is composed of two nonidentical polypeptides termed a-spectrin (3,/, 240,000) and ~-spectrin (Mr 220,000) and is found on the inner surface of the plasma membrane where, in association with actin and several other polypeptides, it forms a dense protein meshwork thought to be involved in the maintenance of cell shape and the structural integrity of the plasma membrane (reviewed in reference 6). Recent resuits have shown that erythroid spectrin and spectrin analogues found in several nonerythroid cell-types have many biochemical and functional properties in common; both form tetramers (2,12,38), bind actin (2,7,8,11,12) and calmodulin (13,19,31), and are associated with the cytoplasmic surface of the plasma membrane (2,8,24,25,29,30,37; for reviews see references 6,23).…”