Human erythrocyte tropomodulin is a novel tropomyosin regulatory protein that binds to the end of erythrocyte tropomyosin and blocks heat-to-tail association of tropomyosin along actin filaments. It has been proposed to play a role in modulating the association of tropomyosin with the spectrin-actin complex in the erythrocyte membrane skeleton. Immunoscreening of a human fetal liver cDNA expression library in lambda gt11, followed by 5'-end extension by polymerase chain reaction from the same library, yielded a composite cDNA sequence of 2665 base pairs (bp). It contains a 34-bp 5'-untranslated region, a 1.6-kilobase (kb) 3'-untranslated region, and a complete open reading frame of 1077 bp that encodes a protein of 359 amino acids with a calculated molecular mass of 40.6 kDa and a pI of 4.8. Authenticity of the tropomodulin cDNA was confirmed by a complete sequence match of 49 predicted amino acids with the sequences of three tryptic peptides of the erythrocyte tropomodulin. The sequence has no internal repeats and no significant homology with any known proteins. Secondary structure predictions indicate that tropomodulin may consist of a series of seven or eight short alpha-helical segments and fold into a somewhat compact shape. The tropomyosin binding activity has been mapped to an N-terminal region containing residues 39-138. Nine independent PCR clones, five from a human reticulocyte cDNA library and four from the fetal liver cDNA library, revealed identical N-terminal 103 amino acids, suggesting that the sequence reported here may also be of erythrocyte tropomodulin. Northern analysis of human reticulocyte RNA showed two hybridizing bands of 2.7 and 1.6 kb, indicating that the 2665-bp cDNA sequence reported here was that of the longer transcript.