J Biol Chem, Vol. 274, Issue 23, 16135-16140, June 4, 1999
In Vitro Unfolding/Refolding of Wild Type Phage P22
Scaffolding Protein Reveals Capsid-binding Domain
Barrie
Greene and
Jonathan
King
From the Department of Biology, Massachusetts Institute of
Technology, Cambridge, Massachusetts 02139
The scaffolding proteins of double-stranded DNA
viruses are required for the polymerization of capsid subunits into
properly sized closed shells but are absent from the mature virions.
Phage P22 scaffolding subunits are elongated 33-kDa molecules that
copolymerize with coat subunits into icosahedral precursor shells and
subsequently exit from the precursor shell through channels in the
procapsid lattice to participate in further rounds of polymerization
and dissociation. Purified scaffolding subunits could be refolded in vitro after denaturation by high temperature or
guanidine hydrochloride solutions. The lack of coincidence of
fluorescence and circular dichroism signals indicated the presence of
at least one partially folded intermediate, suggesting that the protein
consisted of multiple domains. Proteolytic fragments containing the C
terminus were competent for copolymerization with capsid subunits into procapsid shells in vitro, whereas the N terminus was not
needed for this function. Proteolysis of partially denatured
scaffolding subunits indicated that it was the capsid-binding
C-terminal domain that unfolded at low temperatures and guanidinium
concentrations. The minimal stability of the coat-binding domain may
reflect its role in the conformational switching needed for icosahedral
shell assembly.
Copyright © 1999 by The American Society for Biochemistry and Molecular Biology, Inc.