The Dictyostelium myosin IE heavy chain gene encodes a truncated isoform that lacks sequences corresponding to the actin binding site in the tail

We have isolated cDNA and genomic clones which together span the entire coding sequence for the 114.8 kDa heavy chain of Dictyostelium myosin IE (DMIE). The deduced primary sequence reveals a pattern characteristic of all myosins I, i.e., a myosin-like globular head domain fused to a tail domain that shows no similarity to the coiled-coil rod-like tail of type II myosins. The approx. 35 kDa tail domain of DMIE shows some sequence similarity to the membrane interaction region of other myosins I (tail-homology-region 1; TH-1), but lacks completely the sequences that correspond to the second actin binding site (the glycine-, proline- and alanine-rich TH-2 region and the src-like TH-3 region). Therefore, DMIE more closely resembles DMIA (Titus et al. (1989) Cell Regul 1, 55-63), which is also truncated, than DMIB and DMID, both of which possess all three tail homology regions. The similarity between the DMIE and DMIA isoforms extends to their pattern of expression, in which the steady state level of transcript for both genes is highest in vegetative cells and falls gradually after five to ten hours of starvation-induced development. Together, these results have important implications for interpreting and prioritizing gene targeting experiments designed to identify the functions of myosins I in vivo.