Biology Reference
In-Depth Information
orthologue of the Drosophila melanogaster shibire protein (van der Bliek and
Meyerowitz, 1991; Chen et al., 1991). A temperature-sensitive mutant of
shibire was found to cause paralysis when flies were shifted to the restrictive
temperature. Ultrastructural studies of the fly neuromuscular junction
revealed that this phenotype was due to a defect in synaptic vesicle recycling
(Koenig and Ikeda, 1989). These experiments revealed that numerous
vesicular membrane invaginations could form, although final scission was
blocked, and the vesicle buds were arrested at the plasma membrane.
Interestingly, electron dense collars or rings of protein, later shown to include
dynamin, were visible on the necks of the arrested buds. Collectively, these
studies in the fly provided insight into the potential function of dynamin and
suggested that it acted to sever vesicles from the plasma membrane. Over the
past decade, additional studies have demonstrated that there are three
alternatively spliced dynamin (Dyn) genes in mammals, Dyn1, Dyn2 and
Dyn3, that share at least 70% amino acid identity with each other. Further,
the dynamin genes are differentially expressed in adult tissues; Dyn1 is
specifically expressed in the brain, Dyn2 is ubiquitously expressed (Cook et al.,
1994; Sontag et al., 1994) and Dyn3 expression appears to be limited to the
brain, heart, lung and testis (Nakata et al., 1993; Cook et al., 1996).
Domain analysis of the dynamin protein has revealed that each isoform
contains an N-terminal GTPase domain, a pleckstrin homology (PH) domain
that binds phosphoinositide (PI) lipids (mediating the association of dynamin
with membranes) and a C-terminal proline-arginine-rich domain (PRD) that
directly binds to Src homology 3 (SH3) domains (Figure 12.1A). The PRD of
dynamin binds to several proteins that function to promote vesicle budding,
Figure 12.1 (opposite) Dynamin participates in many dynamic actin-based membrane
processes. (A) This basic schematic of the dynamin proteins highlights its three most
important domains. There is an N-terminal GTP hydrolysis domain (GTPase), a centrally
located pleckstrin homology domain (PH) that binds to phosphatidylinositol lipids and a
proline-arginine rich domain (PRD) at the very C-terminus that binds to numerous
proteins, including many that bind and/or regulate the actin cytoskeleton. (B) A fixed cell
expressing PIP5KI and Dyn2-GFP co-stained with rhodamine-phalloidin (actin). Several
comets can be seen in this cell (arrows). The high-magnification inserts nicely demonstrate
that Dyn2-GFP brightly labels the comet 'head'. Studies revealed that Dyn2 has a role in
comet formation and movement. (C) A dividing cell at the late stages of cytokinesis. Dyn2
(red) is highly concentrated at the spindle midzone where it co-localizes with microtubules
(green, co-localization ¼ yellow) and plays a role in the late stages of cytokinesis. The high
magnification insert shows the striking localization of Dyn2 and microtubules at the
midbody matrix. Blue ¼ DAPI. (D) The expression of different Dyn3 spliced-forms induces
alternative phenotypes in cultured hippocampal neurons. Although temporally mature,
neurons expressing Dyn3baa lack dendritic spines and instead are covered with long
filopodia (arrows and D''), which do not assemble functional synapses. As a comparison, a
neuron expressing Dyn3aaa does not exhibit any morphological effect on the normal
formation of short, mushroom-shaped dendritic spines (D'). (A colour reproduction of this
figure can be found in the colour plate section)
Search WWH ::




Custom Search