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suitable object for morphological observations. Most importantly, the ease of
obtaining mutant strains by random 'restriction enzyme mediated insertion'
(REMI) mutagenesis, the recent establishment of regulated expression
(Blaauw et al., 2000) and high copy suppression systems (Robinson and
Spudich, 2000), combined with the fact that the coordinated international
effort to sequence and assemble its 35Mb genome is reaching completion
(http://www.sanger.ac.uk/Projects/D_discoideum/genomic_sequence.shtml),
are enhancing its position as an excellent model organism.
In this chapter, I present a brief review of the recent literature concerned
with the link between class I myosins and actin dynamics, and also proceed to
discuss our recent unpublished results about D. discoideum MyoK in this
context. Finally, as the components of these complex machineries are
evolutionarily conserved, the proposed model is extended to the function of
class I myosins in endocytic membrane tra
cking in mammals.
Structure function analysis of Class I myosins
Class I myosins are divided into about four phylogenetic subclasses (Figure
3.2A; Mooseker and Cheney, 1995; Coluccio, 1997). The subclass I of
ameboid-type myosins can be further subdivided into long-tailed and short-
tailed. All class I myosins identified in most lower eukaryotes, including
Acanthamoeba, S. cerevisiae, S. pombe and Aspergillus have long tails, but in
D. discoideum class I myosins have either long tails (MyoB, MyoC and
MyoD), short tails (MyoA, MyoE and MyoF) or virtually no tail at all
(MyoK) (de la Roche and Cote, 2001). The long tails are 400-450 residues in
size and comprise three tail homology (TH) domains. TH1 is rich in basic
residues, TH2 exhibits a high content of glycine and proline while TH3 is more
commonly referred to as an Src homology 3 (SH3) domain. The short tails are
300-350 residues in size and solely TH1 is recognizable. MyoK is a highly
divergent 94 kDa type I myosin with a very short neck region and a tail only
38 residues in length, but has an insert of *150 amino acids within the motor
domain that bears similarity to TH2 (Yazu et al., 1999; Schwarz et al., 2000).
Phylogenetic analysis shows that D. discoideum MyoC is more closely related
to Acanthamoeba myosin IA, the two S. cerevisiae type I myosins (Myo3p and
Myo5p) and Aspergillus MYOA, while D. discoideum MyoB and MyoD
occupy branches of the phylogenetic tree that contain Acanthamoeba myosin
IB and myosin IC, respectively (Lee et al., 1999). At the moment, only two
homologues of long-tailed amoeboid class I myosins, myosin IE (Sto¨
er et
al., 1995) and IF (Crozet et al., 1997) have been described in mammals, but the
high degree of sequence relatedness is widely accepted as to reflect a high level
of functional conservation. In contrast, whereas short-tailed myosins have not
been identified in other lower eukaryotes than D. discoideum, many examples
