Biology Reference
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Figure 13.5 Microtubules in a Rac1(T17N)-expressing Ptk1 cell. (A) Microtubules in a
live PtK1 cell expressing dominant negative Rac1(T17N). Since these cells are very
contracted it is more dicult to see individual microtubules in the cell periphery. (B)
Differential interference contrast image of the same cell. (C) Kymograph analysis (time
versus distance plot) of the microtubule time-lapse series along the line indicated by the
arrowhead in (A). The arrow in (A) and (C) identifies the same microtubule, which gives
rise to the horizontal dark line in the kymograph showing that it basically did not undergo
retrograde flow
promote microtubule growth by decreasing the catastrophe frequency
(Cassimeris, 2002; Larsson et al., 1997). Accordingly, we have observed
specific phosphorylation of Op18/stathmin by recombinant Pak1 in vitro at
serine 16 and found that Rac1(Q61L)-promoted microtubule growth in Ptk1
cells depends on Pak activity. These observations agree well with the idea that
Rac1 is active in the leading edge of a migrating cell, where microtubule plus
ends exhibit net growth.
However, constitutively acting Pak1 did not have the same effect on
microtubule growth as constitutively active Rac1 (Q61L), indicating that the
Pak activity is necessary but not su
cient for Rac1-mediated microtubule
growth (Wittman et al., 2003). Therefore other mechanisms locally regulating
microtubule dynamics in migrating cells are likely to exist.
