The complete sequence of the Dictyostelium
myosin ID (
DMID) heavy chain
isoform has been determined from
cDNA and genomic clones. Like the DMIB
isoform characterized previously, the
DMID isoform is up-regulated during
starvation-induced chemotactic aggregation, and its 124-kDa heavy chain contains the tail domain sequences that correspond to both the membrane and second actin-binding sites. An antibody that is specific for the
DMID isoform was found to
stain the actin-rich pseudopods at the leading edge of migrating cells.
Protein microsequencing data reveals that the
myosin I isoform localized to leading edge pseudopods in a previous study (Fukui, Y., Lynch, T. J., Brzeska, H., and Korn, E. D. (1989) Nature 341, 328-331) was DMIB, indicating that
DMID and DMIB also colocalize and that both should influence the dynamics of actin-rich cortical structures. This and other data indicate that the
DMID and DMIB
isoforms are closely related and are distinct from the DMIA and DMIE
isoforms, which possess truncated tail domains and are not up-regulated during chemotactic aggregation. Cells in which the
DMID gene was rendered nonfunctional by targeted gene disruption do not show obvious behavioral defects, suggesting that another
myosin I isoform(s) (possibly DMIB) might compensate for
DMID. Finally, Southern blot data indicate that Dictyostelium may contain as many as nine
myosin I isoforms.