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specific proteins and cargoes, the tail is responsible for the specific function
and location of myosins. In addition to the classical involvement of myosin II
in producing contraction power, unconventional myosins are active as cortical
managers, organelle and mRNA transporters, and as regulators of signal
transduction. In particular, class I myosins (Figure 3.2A) form the second
largest group after conventional myosin II, and appear to be essential players
in the establishment and maintenance of cortical tension and related functions
such as motility, endocytosis and exocytosis (Mermall et al., 1998). Finally,
the relevance of myosin for mammalian physiology and pathology was
recently emphasized by the finding that many human and murine genetic
diseases are associated with mutations in myosins (Kabaeva et al., 2002; Berg,
2001; Mermall et al., 1998).
Dictyostelium discoideum as a powerful model organism
D. discoideum is a eukaryotic social amoeba belonging to the crown group of
organisms. Recent studies based on protein phylogeny clearly indicates that
the Amoebozoa group is the closest relative of Metazoa and Fungi, and more
distantly of Plantae (Baldauf et al., 2000). In contrast to yeast, D. discoideum
is famous as a simple model of multicellularity and performs many of the tasks
typical of a higher eukaryote such as chemotaxis, motility, differentiation,
cell-cell contact and ecient endocytosis. This organism also lives as a free
cellular amoeba, feeding by phagocytosis and endocytosis. It possesses an
endomembrane organization similar to higher organisms, and has already
greatly contributed to our understanding of cytoskeleton and cell motility as
well as of developmental and signal transduction pathways (Kessin, 2001). It
is also a potent expression system; easy growth conditions of axenic
laboratory strains give access to high quantities of proteins. As a haploid it
is amenable to molecular genetics, and its relatively large size makes it a
Figure 3.2 (opposite) The family of class I myosins and the structure of members of
subclass I. (A) Class I myosins form the second biggest class after conventional myosin II
and are represented in most organisms except plants and some primitive protozoa. It is
subdivided in four subclasses according to the structure of their neck and tail domains (see
main text for details). (B) Biochemical analysis of class I myosins has demonstrated their
potential function as regulated integrators of membrane and cytoskeleton interactions. The
motor domain of non-metazoan class I myosins is regulated by phosphorylation of the
TEDS site found in the hypertrophic cardiomyopathy loop that contacts actin. The neck
domain binds a variable number of calmodulin-like light chains. The tail domain contains a
polybasic TH1 domain that binds negatively charged phospholipids (+++); a Gly and
Pro-rich TH2 domain that contains secondary actin-binding sites (GPA); an SH3
containing TH3 domain that link to a network of proteins involved in signalling, actin
dynamics and endocytosis
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