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Vol. 8, Issue 10, 2039-2053, October 1997
*Friedrich-Miescher-Laboratorium in der Max-Planck Gesellschaft, myr 5 is an unconventional myosin (class IX) from rat that contains
a Rho-family GTPase-activating protein (GAP) domain. Herein we
addressed the specificity of the myr 5 GAP activity, the molecular mechanism by which GAPs activate GTP hydrolysis, the consequences of
myr 5 overexpression in living cells, and its subcellular localization. The myr 5 GAP activity exhibits a high specificity for Rho. To achieve
similar rates of GTPase activation for RhoA, Cdc42Hs, and Rac1, a
100-fold or 1000-fold higher concentration of recombinant myr 5 GAP
domain was needed for Cdc42Hs or Rac1, respectively, as compared with
RhoA. Cell lysates from Sf9 insect cells infected with recombinant
baculovirus encoding myr 5 exhibited increased GAP activity for RhoA
but not for Cdc42Hs or Rac1. Analysis of Rho-family GAP domain
sequences for conserved arginine residues that might contribute to
accelerate GTP hydrolysis revealed a single conserved arginine residue.
Mutation of the corresponding arginine residue in the myr 5 GAP domain
to a methionine (M1695) virtually abolished Rho-GAP activity.
Expression of myr 5 in Sf9 insect cells induced the formation of
numerous long thin processes containing occasional varicosities. Such
morphological changes were dependent on the myr 5 Rho-GAP activity,
because they were induced by expressing the myr 5 tail or just the myr
5 Rho-GAP domain but not by expressing the myr 5 myosin domain.
Expression of myr 5 in mammalian normal rat kidney (NRK) or HtTA-1 HeLa
cells induced a loss of actin stress fibers and focal contacts with concomitant morphological changes and rounding up of the cells. Similar
morphological changes were observed in HtTA-1 HeLa cells expressing
just the myr 5 Rho-GAP domain but not in cells expressing myr 5 M1695.
These morphological changes induced by myr 5 were inhibited by
coexpression of RhoV14, which is defective in GTP hydrolysis, but not
by RhoI117. myr 5 was localized in dynamic regions of the cell
periphery, in the perinuclear region in the Golgi area, along stress
fibers, and in the cytosol. These results demonstrate that myr 5 has in
vitro and in vivo Rho-GAP activity. No evidence for a Rho effector
function of the myr 5 myosin domain was obtained.
Adolf-Butenandt-Institut/Zellbiologie,
Molecular Biology of the Cell
Vol. 8, 2039-2053, October 1997
Copyright © 1997 by The American Society for Cell Biology
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