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The Carboxy-Terminal Domain Initiates Trimerization of Bacteriophage T4 Fibritin

A. V. Letarov1,2, Yu. Ya. Londer3, S. P. Boudko1, and V. V. Mesyanzhinov1*

1Shemyakin and Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, ul. Miklukho-Maklaya 16/10, Moscow, 117871 Russia; fax: (095) 336-6022; E-mail: vvm@ibch.siobc.ras.ru

2Ivanovsky Institute of Virology, Russian Academy of Medical Sciences, ul. Gamalei 16, Moscow, 123098 Russia

3Bach Institute of Biochemistry, Russian Academy of Sciences, Leninskii pr. 33, Moscow, 117071 Russia

* To whom correspondence should be addressed.

Received November 11, 1998; Revision received March 23, 1999
Bacteriophage T4 fibritin is a triple-stranded, parallel, segmented alpha-helical coiled-coil protein. Earlier we showed that the C-terminal globular domain (foldon) of fibritin is essential for correct trimerization and folding of the protein. We constructed the chimerical fusion protein W31 in which the fibritin foldon sequence is followed by the small globular non-alpha-helical protein gp31 of the T4 phage. We showed that the foldon is capable of trimerization in the absence of the coiled-coil part of fibritin. A deletion mutant of fibritin (NB1) with completely deleted foldon is unable to fold and trimerize correctly. An excess of this mutant protein did not influence the refolding of fibritin in vitro, and the chimerical protein inhibited this process efficiently. Our conclusion is that the trimerization of the foldon is the initial step of fibritin refolding and is followed by the formation of the coiled-coil structure.
KEY WORDS: bacteriophage T4, fibritin, folding, foldon, superhelical protein, coiled coil