Structural gene encoding actin III; expressed only
in the developing thorax, specifically in the indirect flight
muscles (Fyrberg, Mahaffey, Bond, and Davidson, 1983, Cell
33: 115-23; Hiromi and Hotta, 1985, EMBO J. 4: 1681-87).
The only actin expressed in indirect flight muscle (Fyrberg).
Heterozygotes for null mutations or Act88F deficiencies are
flightless; flightlessness is apparently caused by imbalance
between myosin heavy chains and actin III; whereas hemizygosity for either Mhc or Act88F leads to complex myofibrillar
defects and flightlessness, double hemizygotes have nearly
normal fibrillar structure and are able to fly [Beall, Sepanski, and Fyrberg, 1989, Genes Dev. 3: 131-40 (fig.)]. Major
peaks of transcription during pupal stage (Sanchez et al.,
1983). Heterozygotes and to a greater degree, homozygotes and
heteroallelic heterozygotes for antimorphic alleles Act88F4
and Act88F5 show constitutive synthesis of heat-shock proteins, with HSP26 and HSP27 less actively synthesized than
HSP22, HSP70, and HSP84; response to heat shock normal (Hiromi
Dominant flightless allele; actin III replaced by a
truncated polypeptide of 42 kd that is stable and capable of
incorporation into myofibrils; actin II reduced in homozygotes
(Hiromi and Hotta, 1985). Myofibrils in indirect flight muscles of homozygotes severely deranged; sarcomere structure
obliterated; indirect-flight-muscle nuclei enlarged. Skeins
of morphologically normal but highly disorgainzed thick filaments present, but Z discs absent. Thin filaments scarce.
Electron dense material of unknown origin seen in sections.
Wild-type flies transformed with cloned Act4 sequence produces
both the 42-kd and the heat-shock proteins (Hiromi, Okamoto,
Gehring, and Hotta, 1986, Cell 44: 293-301).
Produces half normal amount of actin isoform III;
shows increased synthesis of actin I, normally present in only
trace amounts in indirect flight muscle. Indirect-flight-muscle nuclei enlarged and myofibrils disrupted. Heterozygotes flightless.
Actin III entirely absent from indirect flight muscle in homozygotes; levels of actin II also reduced.
Actin III replaced by a truncated polypeptide of 38
kd; its low concentrations on gels suggests high instability
(Hiromi and Hotta, 1985).
Studied only in combination with rsd at 95.4 on
chromosome 3; not examined in rsd+ background. Wings of
homozygotes held straight up, nearly meeting over thorax;
heterozygotes have wings held normally, but are nearly flightless. Electron microscropy of homozygotes reveals grossly
abnormal indirect-flight-muscle structure; lack thin filaments
and Z discs (Deak, Bellamy, Bienz, Dubuis, Fenner, Gollin,
Rahmi, Ramp, Reinhardt, and Cotton, 1982, J. Embryol. Exp.
Morphol. 69: 61-81). Abnormal protein accumulation observed
in thoraces. Actin III and its ubiquinated derivative, arthrin, absent in Act88F8 homozygotes (Lang et al.); six other
polypeptides, including an indirect-flight-muscle tropomyosin
isoform and two indirect-flight-muscle tropomyosin-related
isoforms, markedly reduced. Homozygotes transformed with
Act88F+ show restoration to approximately normal levels of the
six reduced polypeptides. Accumulation of actin III and arthrin still negative, however; the latter attributed to the
failure of post-translational modification in the presence of
homozygous rsd. Viability and fertility normal.
Homozygotes for Ifm(3)5 and Ifm(3)6 display characteristic departures from wild-type protein patterns in two-dimensional gels (Mogami and Hotta, 1981). Ifm(3)5 homozygotes lack myofibrils; Ifm(3)6 homozygotes have opaque strings
of myofibrils. Both mutants are flightless as heterozygotes.
The myofibrils of Ifm(3)6 heterozygotes are frayed.