Biology Reference
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of such molecules have been extensively investigated in mammalian cells
(Mooseker and Cheney, 1995; Coluccio, 1997).
It has been generally well established that TH1 binds anionic phospholipids,
while TH1 and TH2, bind F-actin in an ATP-independent manner (de la
Roche and Cote, 2001). As a consequence, all class I myosins probably have
the potential to associate to a membrane and to translocate F-actin along its
surface, as well as to bind two actin filaments and slide them relative to each
other (Figure 3.2B). Intriguingly, D. discoideum MyoK has many of the same
properties but uses a different molecular architecture. It has no TH1, TH2 and
TH3 tail domains, but has a C-terminal farnesylation in place of
phospholipid-binding TH2, and harbours a TH2-like insert within the
motor domain (Schwarz et al., 2000; and see below). The long tails of
ameboid class I myosins may fold so as to position TH1 and TH2 side by side
and the SH3 domain adjacent to the neck (Lee et al., 1999). This location of
the SH3 domain close to the motor might explain why the MgATPase activity
of rat myosin IE is stimulated by proteolytic removal of the SH3 domain or by
binding of an antibody to the C-terminus (Sto¨
er and Ba¨ hler, 1998).
Investigation of D. discoideum MyoK
MyoK was discovered as a result of an exhaustive screening for completion of
the myosin repertoire in D. discoideum, which also revealed MyoM (Schwarz
et al., 1999). MyoM is the first myosin to carry a Rac GTPase activator
domain in its tail and is involved in actin cytoskeleton reorganization (Geissler
et al., 2000). MyoK was characterized at the levels of cDNA and genomic
sequences and organization, and protein expression and localization. MyoK
shares structural features with class I myosins, but is distinguished by an
unusual architecture (Figure 3.3; Schwarz et al., 2000). MyoK is one of the
smallest myosins found to date, and its most striking feature is the virtual
absence of a tail or cargo-binding domain. MyoK ends with a -CLIQ
sequence, corresponding to a so-called -CAAX motif, a known protein
farnesylation signal (Figure 3.3A) that was shown to be su
cient to target a
GFP fusion protein to the plasma membrane (E. Schwarz and T. Soldati,
unpublished results). It is unclear whether, as all other myosins studied so far,
MyoK carries a light chain bound to its short neck that does not carry any
canonical IQ motif. As an apparent form of 'compensation', MyoK has a 150
residues insertion in its surface loop 1. This insertion is extremely rich in Gly,
Pro and Arg (GPR), a composition similar to the TH2 tail domains of other
class I myosin (Figure 3.3B), where it has been shown to contain F-actin
binding sites. Moreover, the GPR-loop has about 40% identity (over 60%
homology) with the Pro-rich domain of some WASp proteins (Figure 3.3C).
Closer analysis revealed that
it contains a variety of Pro-rich motifs
