We have cloned and completely sequenced a gene encoding the heavy chain of Dictyostelium
myosin I. Like the
myosin I molecules from Acanthamoeba, the Dictyostelium
myosin I heavy chain is composed of a globular head domain fused to a 45-kDa
glycine-,
proline-, and
alanine-rich carboxyl-terminal domain, rather than the coiled-coil rod domain of conventional
myosins. Comparisons of the Dictyostelium
myosin I heavy-chain amino acid sequence with those of the Acanthamoeba
myosins I reveal that they are highly similar throughout, including the unconventional carboxyl-terminal domains. The Dictyostelium
myosin I gene is expressed in growing cells as a 3600-nucleotide
mRNA. Measurements of the steady-state level of this
mRNA at different times during
starvation-induced aggregation and development are consistent with a role for
myosin I in chemotaxis and aggregation. Generation of Dictyostelium cells lacking
myosin I by gene disruption and/or
antisense RNA production should provide a way to test directly the role of this nonfilamentous
myosin in cell motility. These experiments will be simplified by the fact that Southern blot analyses of Dictyostelium genomic
DNA are consistent with there being a single
myosin I heavy-chain gene.