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

Abstract

Erythrocyte tropomodulin (E-Tmod), a globular protein of 359 residues, is highly expressed in the erythrocyte, heart and skeletal muscle. By binding to the N-terminus of tropomyosin (TM) and actin, E-Tmod blocks the elongation and depolymerization of the actin filaments at the pointed end. In erythrocytes, the E-Tmod/TM complex contributes to the formation of the short actin protofilament, which in turn defines the geometry of the membrane skeleton. 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 genomic library screening and PCR-based genomic walking we have now cloned the mouse E-Tmod gene, whose coding region spans approximately 60kb containing nine exons and eight introns. The human E-Tmod gene obtained by PCR has an identical 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' exons may give rise to several sanpodo isoforms. In a Tmod-like gene of C. elegans, exons 2-3 are 'fused', boundaries of exons 1, 7, 8, and 9 are conserved and exon/intron junctions of exons 4, 5 and 6 are shifted by a few residues. Analyses of 15 Tmod members from six species show no insertions or deletions 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 additional 86bp untranslated cDNA sequence further upstream from exon 1. Thus, alternative promoters may provide a possible mechanism for tissue-specific expression and regulation of E-Tmod. This study is the first to report the exon organization of E-Tmod genes, which allows their regulation, manipulation, and disease relevance to be further investigated.