The human RAD52 protein plays an important role in the earliest stages of c
hromosomal double-strand break repair via the homologous recombination path
way. Individual subunits of RAD52 associate into seven-membered rings. Thes
e rings can form higher order complexes. RAD52 binds to DNA breaks, and rec
ent studies suggest that the higher order self-association of the rings pro
motes DNA end joining. Monomers of the RAD52(1-192) deletion mutant also as
sociate into ring structures but do not form higher order complexes. The th
ermal stability of wild-type and mutant RAD52 was studied by differential s
canning calorimetry. Three thermal transitions (labeled A, B, and C) were o
bserved with melting temperatures of 38.8, 73.1, and 115.2 degreesC. The RA
D52(1-192) mutant had only two thermal transitions at 47.6 and 100.9 degree
sC (labeled B and C). Transitions were labeled such that transition C corre
sponds to complete unfolding of the protein. The effect of temperature and
protein concentration on RAD52 self-association was analyzed by dynamic lig
ht scattering. From these data a four-state hypothetical model was develope
d to explain the thermal denaturation profile of wild-type RAD52. The three
thermal transitions in this model were assigned as follows. Transition A w
as attributed to the disruption of higher order assemblies of RAD52 rings,
transition B to the disruption of rings to individual subunits, and transit
ion C to complete unfolding. The ring-shaped quatenary structure of RAD52 a
nd the formation of higher ordered complexes of rings appear to contribute
to the extreme stability of RAD52. Higher ordered complexes of rings are st
able at physiological temperatures in vitro.