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Figure 5.1 A general scheme of cell motility regulation and the role of mDia1. External
'motogenic' factors stimulate tyrosine kinase receptors, G-protein coupled receptors, or
other signalling pathways leading to an alteration of activities of a subset of Rho family
GTPases. This, in turn, triggers a subset of targets, which induce cytoskeletal re-
organization. The cytoskeletal changes are required for formation/release of cell-matrix
adhesions, and cytoskeletal and adhesion reorganization together produce cell movements.
Adhesion-dependent signals, in turn, affect the Rho GTPase activities, closing the feedback
loop. mDia1, one of immediate Rho targets, is discussed in detail in the present review. It
affects both actin and microtubules, resulting in reorganization of both the cytoskeleton
and focal adhesions
'dynamic instability' (see for actin, Littlefield and Fowler (2002), and for
microtubules, Waterman-Storer and Salmon (1997)). Fibre assembly and
disassembly are regulated by a variety of specific proteins, and the principles of
this regulation have many common features (see Pollard and Borisy, 2003 for
actin, Heald and Nogales, 2002 for microtubules). The existence of a particular
mechanism of actin regulation usually indicates that a parallel mechanism is
used by microtubules, and vice versa. For example, the molecular complexes
that nucleate microtubules and actin filaments include, as essential
components, proteins that are highly homologous to the subunits of
the corresponding fibres. Thus, the principal component of microtubule-
nucleating complexes is gamma-tubulin (Moritz and Agard, 2001), while the
core of the complex that nucleates actin filaments (Arp2/3 complex) comprises
the a ctin- r elated p roteins, Arp2 and Arp3 (Pollard and Beltzner, 2002).
Despite the apparent similarity in the general organization of the two
cytoskeletal systems, cells use actin and microtubule units to perform
different, albeit complementary functions. An illuminating example of
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