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regulated actin-crosslinker. Nevertheless, as MyoK is not abundant, we
hypothesized that it works less mechanically than catalytically and might
have a distinctive role in cortical management (Schwarz et al., 2000).
Phenotypes resulting from manipulation of class I myosins
Immunolocalization studies of class I myosins in amoeba (de la Roche and
Cote, 2001; Ma et al., 2001) and in higher eukaryotes (Kalhammer and
Bahler, 2000; Berg et al., 2001) visualized their association with dynamic
regions of the cell cortex, such as the leading pseudopod of migrating cells and
endocytic structures, microvillosities, cell-cell junctions (Sto¨
er et al., 1998)
and sites of particle ingestion (Diakonova et al., 2002). In less plastic fungal
cells, the major site of myosin I localization is within the actin cortical patches
(Win et al., 2002).
Gene disruption studies have implicated D. discoideum class I myosins in a
palette of actin-based processes, many of which were interpreted as a result of
the lipid-binding and actin-binding properties of TH1 and TH2, leading to a
contraction of the cortical actin network or transport of vesicles along actin
filaments (Ma et al., 2001; de la Roche and Cote, 2001). Cells lacking either
MyoA or MyoB moved with reduced velocity, formed more pseudopods, and
turned more frequently than wild-type (Wessels et al., 1991, 1996; Titus et al.,
1993). Double mutants (A-/B-, B-/C-) showed that these myosins contribute
to the generation of cortical tension (Dai et al., 1999), suggesting that both
long-tailed and short-tailed myosins play a role in the organization of the actin
cytoskeleton. Cell movement, macropinocytosis and phagocytosis all use the
actin cytoskeleton to extend membrane protrusions and it has been shown
that myosin motors are also involved in these processes. D. discoideum MyoB
has been located to the phagocytic cup (Fukui et al., 1989), and membrane
rues (Novak et al., 1995). Cells deficient in MyoB have a reduced rate of
phagocytosis (Jung and Hammer, 1990; Jung et al., 1996), whereas
overexpression of MyoB resulted in decreased macropinocytosis (Novak
and Titus, 1997). Single MyoC mutants have a decreased initial rate of fluid-
phase uptake (Jung et al., 1996), and the slow growth of various double and
triple mutants (A-/B-, B-/C-, B-/D-, B-/C-/D-) was interpreted as additive
impairments of myosin I function in fluid-phase uptake (Jung et al., 1996;
Novak et al., 1995). These results led to the proposition that class I myosins
share partially overlapping but mainly non-redundant functions in endo-
cytosis. Findings in Acanthamoeba (Baines et al., 1992) and Entamoeba
histolytica (Voigt et al., 1999) are in excellent agreement with these
observations.
To study the function of MyoK in vivo, we analysed the phenotypic
alterations in myoK null cells and MyoK overexpressing cells. We observed
