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Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710
Dictyostelium myoB, a member of the myosin I family of motor proteins, is important for controlling the formation and retraction of membrane projections by the cell's actin cortex (Novak, K.D., M.D.
Peterson, M.C. Reedy, and M.A. Titus. 1995. J. Cell
Biol. 131:1205-1221). Mutants that express a three- to
sevenfold excess of myoB (myoB+ cells) were generated to further analyze the role of myosin I in these
processes. The myoB+ cells move with an instantaneous velocity that is 35% of the wild-type rate and exhibit a 6-8-h delay in initiation of aggregation when
placed under starvation conditions. The myoB+ cells
complete the developmental cycle after an extended
period of time, but they form fewer fruiting bodies that
appear to be small and abnormal. The myoB+ cells are
also deficient in their ability both to form distinct F-actin filled projections such as crowns and to become
elongate and polarized. This defect can be attributed to
the presence of at least threefold more myoB at the
cortex of the myoB+ cells. In contrast, threefold overexpression of a truncated myoB that lacks the src homology 3 (SH3) domain (myoB/SH3
cells) or myoB in
which the consensus heavy chain phosphorylation site
was mutated to an alanine (S332A-myoB) does not disturb normal cellular function. However, there is an increased concentration of myoB in the cortex of the
myoB/SH3
and S332A-myoB cells comparable to that
found in the myoB+ cells. These results suggest that excess full-length cortical myoB prevents the formation of
the actin-filled extensions required for locomotion by
increasing the tension of the F-actin cytoskeleton and/
or retracting projections before they can fully extend.
They also demonstrate a role for the phosphorylation
site and SH3 domain in mediating the in vivo activity of
myosin I.
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