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* Department of Molecular Biology, School of Science, Nagoya University, Nagoya 464-01, Japan; Chlamydomonas flagellar inner-arm dynein
consists of seven subspecies (a-g), of which all but f
contain actin as subunits. The mutant ida5 and a new
strain, ida5-t, lack four subspecies (a, c, d, and e). These
mutants were found to have mutations in the conventional actin gene, such that its product is totally lost;
ida5 has a single-base deletion that results in a stop
codon at a position about two-thirds from the 5 Department of Biological
Sciences, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan; and § National Institute for Basic
Biology, Okazaki 444, Japan
end of
the coding region, and ida5-t lacks a large portion of the
entire actin gene. Two-dimensional gel electrophoresis
patterns of the axonemes and inner-arm subspecies b
and g of ida5 lacked the spot of actin (isoelectric point
[pI] = ~5.3) but had two novel spots with pIs of ~5.6
and ~5.7 instead. Western blot with different kinds of
anti-actin antibodies suggested that the proteins responsible for the two novel spots and conventional actin are different but share some antigenicity. Since
Chlamydomonas has been shown to have only a single
copy of the conventional actin gene, it is likely that the
novel spots in ida5 and ida5-t originated from another
gene(s) that codes for a novel actin-like protein(s)
(NAP), which has hitherto been undetected in wildtype cells. These mutants retain the two inner-arm subspecies b and g, in addition to f, possibly because NAP
can functionally substitute for the actin in these subspecies while they cannot in other subspecies. The net
growth rate of ida5 and ida5-t cells did not differ from that of wild type, but the mating efficiency was greatly
reduced. This defect was apparently caused by deficient
growth of the fertilization tubule. These results suggest
that NAP can carry out some, but not all, functions performed by conventional actin in the cytoplasm and
raise the possibility that Chlamydomonas can live without ordinary actin.
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