Tropomyosin isoform 5b is expressed in human erythrocytes: implications oftropomodulin-TM5 or tropomodulin-TM5b complexes in the protofilament and hexagonal organization of membrane skeletons

The human erythrocyte membrane skeleton consists of hexagonal lattices with junctional complexes containing F-actin protofilaments of approximately 33 -37 nm in length. We hypothesize that complexes formed by tropomodulin, a g lobular capping protein at the pointed end of actin filaments, and tropomyo sin (TM), a rod-like molecule of approximately 33-35 nm, may contribute to the formation of protofilaments, We have previously cloned the human tropom odulin complementary DNA and identified human TM isoform 5 (hTM5), a produc t of the gamma-TM gene, as one of the major TM isoforms in erythrocytes, We now identify TM5b, a product of the alpha-TM gene, to-be the second major TM isoform, TM5a, the alternatively spliced isoform of the (U-TM gene, whic h differs by 1 exon and has a weaker actin-binding affinity, however, is no t present. TM4, encoded by the delta-TM gene, is not present either. In sod ium dodecyl sulfate-polyacrylamide gel electrophoresis, hTM5 comigrated wit h the slower TM major species in erythrocyte membranes, and hTM5b comigrate d with the faster TM major species. TM5b, like TM5, binds strongly to tropo modulin, more so than other TM isoforms, The 2 major TM isoforms, therefore , share several common features: They have 248 residues, are approximately 33-35 nm long, and have high affinities toward F-actin and tropomodulin. Th ese common features may be the key to the mechanism by which protofilaments are formed. Tropomodulin-TM5 or tropomodulin-TM5b complexes may stabilize F-actin in segments of approximately 33-37 nm during erythroid terminal dif ferentiation and may, therefore, function as a molecular ruler. TM5 and TM5 b further define the hexagonal geometry of the skeletal network and allow a ctin-regulatory functions of TMs to be modulated by tropomodulin, (C) 2000 by The American Society of Hematology.