Aggregation, Protein
31
and coworkers. The authors showed that
during the
in vitro
refolding of a mixture
of two proteins, tailspike endorhamnidase
and coat protein from phage P22, no het-
erogeneous aggregates were formed. Tail-
spike endorhamnidase is a thermostable
trimer whose folding intermediates are
thermolabile and either undergo produc-
tive folding or form multimeric aggregates
(Fig. 3). The P22 coat protein, which com-
prises the capsid shell of phage P22, yields
either a correct fold or ‘‘off-pathway’’ aggre-
gates upon refolding. Both proteins were
intensively studied by King and cowork-
ers who ±rst denatured the two proteins
in urea and then chose refolding con-
ditions such that aggregation competes
with correct folding. Folding and soluble
aggregates of the two proteins were charac-
terized either separately or mixed together.
No heterogeneous aggregates were found,
clearly indicating that only self-association
of transient refolding molecules occurs in
the formation of soluble multimers.
One mechanism that accounts for the
formation of aggregates during refolding
of multidomain proteins is domain swap-
ping. This was ±rst suggested by Monod
and later proposed by Goldberg and col-
leagues to account for the formation of
aggregates during the refolding of trypto-
phanase. The concept was foreshadowed
by the results of Crest±eld and cowork-
ers in 1962. From their experiments based
on chemical modi±cation of bovine pan-
creatic ribonuclease, the authors proposed
that the dimer is formed by exchanging
the N-terminal fragments. The term
3D
domain swapping
was introduced in 1994
by Bennett and coworkers to describe the
structure of a diphtheria toxin dimer. The
mechanism involves the replacement of
one domain of a monomeric protein by the
same domain of an identical neighboring
Aggregate
=
inclusion body
Suppressor mutation
action
(I*)
(I)
40
°
C
30
°
C
tsf
Substitution
Early-folding
intermediate
Native
tailspike
Protrimer
Fig. 3
The folding pathway of the P22 tailspike protein. (From Mitraki, A., King, J. (1992)
FEBS Lett.
307
, 20–25; reproduced with permission.)
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