Genomic organization of mouse and human erythrocyte tropomodulin genes encoding the pointed end capping protein for the actin filaments

Erythrocyte tropomodulin (E-Tmod), a globular protein of 359 residues, is h ighly expressed in the erythrocyte, heart and skeletal muscle. By binding t o the N-terminus of tropomyosin (TM) and actin, E-Tmod blocks the elongatio n and depolymerization of the actin filaments at the pointed end. In erythr ocytes, the E-Tmod/TM complex contributes to the formation of the short act in protofilament, which in turn defines the geometry of the membrane skelet on. In juvenile mice, over-expression of E-Tmod is associated with dilated cardiomyopathy. We have previously cloned the human E-Tmod cDNA, identified its TM-binding region, and mapped its gene to chromosome 9q22. Through gen omic library screening and PCR-based genomic walking we have now cloned the mouse E-Tmod gene, whose coding region spans similar to 60 kb containing n ine exons and eight introns. The human Tmod gene obtained by PCR has an ide ntical exon-intron organization. In sanpodo, a Tmod homologue in Drosophila , the exon boundaries are also conserved except that exons 2-5 and 6-7 are 'fused' and alternative splicing of two additional 5' exons and the 3' exon s may give rise to several sanpodo isoforms. In a Tmod-like gene of C. eleg ans, exons 2-3 are 'fused', boundaries of exons 1, 7, 8, and 9 are conserve d and exon/intron junctions of exons 4, 5 and 6 are shifted by a few residu es. Analyses of 15 Tmod members from six species show no insertions or dele tions of residues in the region of exons 6 and 7. A 5' rapid amplification of cDNA ends reveals that mouse E-Tmod transcripts obtained from embryonic stem cells, skeletal muscle and heart, but not smooth muscle, contain an ad ditional 86 bp untranslated cDNA sequence further upstream from exon 1. Thu s, alternative promoters may provide a possible mechanism For tissue-specif ic expression and regulation of E-Tmod. This study is the first to report t he exon organization of E-Tmod genes, which allows their regulation, manipu lation, and disease relevance to be further investigated. (C) 2000 Publishe d by Elsevier Science B.V. All rights reserved.