download report - Istituto Pasteur

P a r t i c i p a n t s :
Stefano Cacchione, researcher; Sabrina Pisano, post-doc fellow;
Alessandra Galati, Emanuela Micheli, PhD students.
C o l l a b o r a t i o n s :
Dipartimento di Chimica, Sapienza-Università di Roma (Prof.
Armandodoriano Bianco, Prof. Pasquale De Santis, Prof.
Giancarlo Ortaggi, Dr. Marco Franceschin, Dr. Anita
Scipioni); Dipartimento di Chimica delle Sostanze Naturali,
Università di Napoli “Federico II” (Prof. Luciano Mayol, Dr.
Michela Varra); Medical Research Council, Laboratory of
Molecular Biology, Cambridge, UK (Dr. Daniela Rhodes, Dr.
Linda Chapman); Laboratory of Molecular and Cellular Biology,
ENS, Lyon (Prof. Eric Gilson).
Report of Activity
Telomeres are the special nucleoprotein structures
that protect chromosome ends from both recombination
and degradation. In humans, telomeres consist
of TTAGGG repeats about 10 kbp long, ending in a
3’ G-rich overhang. As a consequence of incomplete
DNA replication at chromosome termini, telomeres
progressively shorten in replicating cells, till they
reach a critical length that leads to cellular senescence.
In germ line cells, telomere loss is counteracted
by the reverse transcriptase enzyme telomerase,
which adds telomeric repeats to the 3’ ends of the
chromosomes using as template its RNA moiety.
Both telomere length regulation and the maintenance
of a correct end-capping structure are essential
for cell and organism survival. Most tumors
show aberrant telomerase expression levels, that
allow cells to divide indefinitely.
The long human telomeres are organized in tightly
packed nucleosomes separated by 10-20 bp of linker
DNA. Two proteins, TRF1 and TRF2, bind to doublestranded
TTAGGG repeats, whereas POT1 recognizes
single-stranded G-rich overhangs. Telomere
structure is still poorly defined. A sound model is the
Principal investigator: Maria Savino
Professor of Biophysical Chemistry
Dipartimento di Genetica e Biologia Molecolare
Tel: (+39) 06 49912238; Fax: (+39) 06 4440812
maria.savino@uniroma1.it
91
Molecular recognition in biomolecules - AREA 4
Structural and superstructural features of human telomeric
chromatin
t-loop, a closed protective structure in which the 3’
overhang circles back and insert into the upstream
duplex telomeric DNA region. The G-rich singlestranded
extension can also fold to form a structure
named G-quadruplex, a four-stranded structure stabilized
by cations at physiological concentrations. The
emerging view is that telomeres are dynamic structures
that interconvert among different structures
along with the cell cycle and differentiation.
Whereas the roles played by telomere-binding proteins
in the regulation of telomere length and stability have
been widely studied, little is known about the involvement
of nucleosomes in telomere structure and functions,
and about the interplay between nucleosomes
and specific telomeric proteins. In the past years, we
demonstrated that telomeric nucleosomes are the least
stable nucleosomes so far studied and occupy multiple
isoenergetic positions, as a consequence of the 6 bp
telomeric DNA periodicity, which is out of phase with
the DNA double helical repeat, Furthermore, AFM
studies on nucleosomal arrays reconstituted in vitro
showed that the very short spacing of human telomeric
nucleosomes (157 bp) depends mainly on the intrinsic
physical properties of telomeric DNA. Finally, we
found that hTRF1 is able to specifically recognize
telomeric binding sites located within nucleosomes,
inducing alterations in nucleosome structure without
dissociation of histone subunits. All these findings
suggest that nucleosomes may play a relevant role in
telomere function and dynamics.
In the past two years we focused on two main issues:
1) The role of nucleosomes in telomere dynamics
and their interplay with TRF proteins; 2) Human
telomere G-quadruplex structures as targets for
anticancer strategies.
Intrinsic and induced mobility of telomeric
nucleosomes
Chromatin remodeling is a major player in the regulation
of biological processes, through the action of
ATP-dependent remodeling complexes. By means of