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morphological changes of the cortical actin cytoskeleton accompanied by
impairments in the rate of phagocytosis and in chemotactic motility. Cells
deficient in one or more myosin Is extend more lateral pseudopods during
motility, exhibit decreased rates of pinocytosis and phagocytosis and have a
lower cortical tension (Jung et al., 1996; Novak et al., 1995; Schwarz et al.,
2000; Dai et al., 1999) while overexpression of some class I myosins augments
cortical tension and restricts extension of actin-filled protrusions (Novak and
Titus, 1997; Schwarz et al., 2000). The phenotype of myoK null cells was the
more dramatic of all single myosin I knock outs reported so far (Dai et al.,
1999), especially with regard to the significant effects on cortical tension.
Indeed, myoK null cells had a 24% lower cortical tension than wild-type cells,
and myoK overexpresses a 70% higher cortical tension (Schwarz et al., 2000).
This is remarkable considering the high potential degree of
redundancy between the seven class I myosins of D. discoideum.
Because of their sheer number, it is practically impossible to disrupt all
seven class I myosins genes in D. discoideum, but in S. cerevisiae it was
shown that mutants lacking both Myo3p and Myo5p exhibit a strong
growth defect, accumulate intracellular vesicles and are severely impaired in
endocytosis (Geli and Riezman, 1996; Goodson et al., 1996). MYOA is
required for the viability of Aspergillus and is involved in the generation of
cell polarity and focal secretion at the tip of growing hyphae (McGoldrick
et al., 1995; Osherov et al., 1998). Several more recent studies highlight the
importance of the SH3 domain and thereby emphasize another aspect of
myosin I function. The defects in growth, endocytosis and actin organization
exhibited by D. discoideum cells lacking MyoA and MyoB can be fully
rescued by wild-type MyoB but not by MyoB missing its SH3 domain
(MyoBDSH3) (Novak and Titus, 1998). Furthermore, overexpression of
MyoBDSH3 does not generate the defects associated with overexpression of
wild-type MyoB (Novak and Titus, 1997). In S. cerevisiae, the severe defects
resulting from Myo3p/Myo5p double knockout can be complemented by
either Myo3p or Myo5p lacking TH2 but not lacking their SH3 domain
(Anderson et al., 1998; Evangelista et al., 2000). In contrast, Aspergillus
MYOA appears to function even when its SH3 domain is deleted (Osherov
et al., 1998).
Class I myosins and the actin dynamics connection
SH3 domains in general are found in an impressive number of factors involved
in signal transduction, actin dynamics and endocytic membrane tra cking
(Schafer, 2002; Warren et al., 2002; Dewar et al., 2002; Mochida et al., 2002;
Soulard et al., 2002; Bon et al., 2000), and have been shown to interact
specifically with either one or a few proline-rich motifs of either class I
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