Conférence
de Carlos BUSTAMANTE (Lawrence Berkeley National Laboratory et
Howard Hughes Medical Institute, University of California,
Berkeley, USA) le vendredi 19 juin 2009 à
11 h 30, IBSM (salle de conférences Jacques Senez), Campus
Joseph Aiguier, Bt IM, CNRS, Marseille : « Grabbing the cat
by the tail: discrete steps by a DNA packaging motor and the
inter-subunit coordination in a ring-ATPase ».
Carlos BUSTAMANTE est Membre de l'Académie des Sciences
des États-Unis, et est pionnier des méthodes de
manipulation et détection sur molécules uniques
(optical tweezers, single-molecule þuorescence microscopy).
Ces méthodes permettent l'étude des divers moteurs
DNA-protéine (hélicases, translocases, ring ATPases)
et à l'avenir pourraient permettre l'étude des
protéases et chaperones.
Résumé
Coordination between subunits is crucial for the proper functioning
of multi-component molecular systems. Single-molecule studies now
allows glimpses into the mechanism used by subunits of one such
system, a ring ATPase. Homomeric ring ATPases are found in all
forms of life and are involved in many processes such as chromosome
segregation, protein unfolding and ATP synthesis. How these enzymes
coordinate ATP hydrolysis and mechanistic function is largely
unknown. One such ring ATPase from a bacteriophage, Phi29, helps
load the dsDNA genome into the viral shell. Bustamante and his
team, using high precision, single molecule assay, involving
dual-beam "optical tweezers" have just shown that this
Þve-membered motor packages the DNA in 10-base-pair bursts,
each consisting of four individual 2.5-bp steps corresponding to
the hydrolysis of a single ATP. They thus provide the Þrst
direct measurement of a single enzymatic cycle by this ATPase,
revealing an unexpected form of coordination between the subunits.
The non-integral step size detected is unprecedented, and raises
intriguing mechanistic questions about ATP hydrolysis within rings,
and the interactions of the ring with DNA.