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כיצד מנועים ביולוגיים מצליחים להסיע חומרים חיוניים לאורכו ולרוחבו של התא,
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A set of mutant yeast strains allows researchers to identify structural elements
that help motor proteins to get moving
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Cells are crisscrossed by microtubules, protein cables that provide
essential infrastructure and serve as ‘highways’ for moving molecular
cargoes. Motor proteins, such as kinesin that travels along microtubules
via a multi-step ‘walking’ mechanism, effectively drive this transport.
The broad strokes of this process are well understood generally, but
new work from Etsuko Muto and Seiichi Uchimura of the RIKEN Brain
Science Institute in Wako in collaboration with physicists at Waseda
University, Tokyo, has revealed valuable new details about how microtubule
interactions facilitate kinesin movement.
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Kinesin is associated with the nucleotide molecule adenosine diphosphate
(ADP) when it first binds microtubules, after which it undergoes a structural
change that triggers release of ADP and enables interaction with adenosine
triphosphate (ATP). Subsequent enzymatic processing of ATP into ADP triggers
additional structural changes, causing kinesin to move forward along the
microtubule while also returning the protein to its initial ADP-bound state.
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Microtubules are composed of dimers of the protein α- and β-tubulin, but
eukaryotic cells can have numerous different tubulin subtypes, making it
challenging to investigate molecular-level details of kinesin–tubulin interaction.
To overcome this problem, Muto and Uchimura developed yeast strains that
express only a single subtype each of α- and β-tubulin, thus enabling simple
screening of the effects of individual tubulin mutations. In their most recent work,
they have used this approach to extensively characterize points of interaction
between kinesin and microtubules by generating 36 yeast strains with
individual mutations in either tubulin subunit.
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Their data suggest that α-tubulin is primarily responsible in the initial association
with kinesin-ADP, with β-tubulin providing important stabilizing interactions
following the release of ADP . The researchers were particularly surprised
to note that mutations targeting one highly conserved glutamate (E415) in α-tubulin
caused a five-fold reduction in kinesin enzymatic activity, apparently by impairing
binding-induced release of ADP. “Our results indicate that kinesin binding to residue
E415 in α-tubulin transmits a signal to the kinesin nucleotide pocket, triggering its
conformational change and leading to release of ADP,” explains Muto. “I did not expect
that residues in α-tubulin would play such an important role.”
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In future studies, Muto and Uchimura hope to further dissect the amino acid network
that communicates these structural changes across the kinesin protein. Since microtubules
play a key role in diverse cellular functions beyond molecular transport, Muto believes
that their mutational analysis strategy should also offer a powerful tool for studying
processes ranging from the separation of chromosome pairs during cell division to
cilia-mediated cell propulsion.
