Chlamydomonas
639
Fig. 8
Cross section of the
C. reinhardtii
flagellum showing the
nine outer doublets with inner and outer dynein arms and the
radial spokes extending toward the central pair of tubules; bar
indicates 0.05 m. (Reproduced with permission from Witman, in
Ciliary and Flagellar Membranes
, R.A. Bloodgood, Ed., Plenum
Press, New York, 1990, pp. 1–30.)
encoding a 69 kDa intermediate chain,
thus showing that a defect in this chain can
interfere with outer-arm assembly. The
α
-
heavy chain does not appear to be required
for stable outer-arm assembly because a
mutant missing the
α
-chain and its associ-
ated 16 kDa light chain is still able to form
a partially functional truncated outer arm.
Two morphologically different types of in-
ner arm have been identiFed. However,
biochemical and genetic studies indicate
the existence of at least Fve different types
of inner arms, consisting of homodimers
and heterodimers of six inner-arm heavy
chains, and of smaller polypeptides. Mu-
tants lacking the outer arms or some of
the inner arms are still mobile, showing
that none of these arms is essential for
interdoublet sliding. Since the former are
affected in flagellar beat frequency and the
latter in the waveform, however, it is clear
that functional differences exist between
the arms.
The most abundant flagellar proteins are
the
α
-and
β
-tubulins, each of which is en-
coded by two genes. Both
β
-genes encode
identical proteins, while the products of
the two
α
-genes differ by two amino acids.
Bes
idesthetubu
l
ins
,asmanyas250to
300 distinct flagellar polypeptides can be
resolved. Analysis of many paralyzed mu-
tants has revealed that some are deFcient
in speciFc axonemal components. The de-
fects observed in these mutants have been
found to cosegregate with the mutant phe-
notype. Mutants lacking the central pair of
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