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 entir e coding sequence for the 114.8 kDa heavy chain of Dictyostelium myosi n IE (DMIE). The deduced primary sequence reveals a pattern characteri stic 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 m yosins I (tail-homology-region 1; TH-1), but lacks completely the sequ ences that correspond to the second actin binding site (the glycine-, proline- and alanine-rich TH-2 region and the src-like TH-3 region). T herefore, DMIE more closely resembles DMIA (Titus et al. (1989) Cell R egul 1, 55-63), which is also truncated, than DMIB and DMID, both of w hich possess all three tail homology regions. The similarity between t he DMIE and DMIA isoforms extends to their pattern of expression, in w hich the steady state level of transcript for both genes is highest in vegetative cells and falls gradually after five to ten hours of starv ation-induced development. Together, these results have important impl ications for interpreting and prioritizing gene targeting experiments designed to identify the functions of myosins I in vivo.