Mapping the tropomyosin isoform 5 binding site on human erythrocyte tropomodulin: Further insights into E-Tmod/TM5 interaction

Actin protofilaments in the erythrocyte membrane skeleton are uniformly ∼37 nm. This length may be in part attributed to a “molecular ruler” made of erythrocyte tropomodulin (E-Tmod) and tropomyosin (TM) isoforms 5 or 5b. We previously mapped the E-Tmod binding site to TM5 N-terminal heptad repeat residues “a” (I7, I14), “d” (V10) and “f” (R12). We now map the TM5 binding site to E-Tmod residues at L116, E117 and/or E118 by identifying among 35 deletion clones and a series of point mutations that no longer bind to human TM5 and rat TM5b. Upstream residues 71–104 contain an actin binding site. The N-terminal “KRK ring” may participate in balancing electrostatic force with hydrophobic interaction in dimerization of TM and its binding to E-Tmod.