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J. Biol. Chem., Vol. 282, Issue 5, 2821-2831, February 2, 2007

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Characterization of a Novel Intramolecular Chaperone Domain Conserved in Endosialidases and Other Bacteriophage Tail Spike and Fiber Proteins^*

From the Abteilung Zelluläre Chemie, Zentrum Biochemie, Medizinische Hochschule Hannover, Carl-Neuberg-Strasse 1, 30625 Hannover, Germany

Folding and assembly of endosialidases, the trimeric tail spike proteins of Escherichia coli K1-specific bacteriophages, crucially depend on their C-terminal domain (CTD). Homologous CTDs were identified in phage proteins belonging to three different protein families: neck appendage proteins of several Bacillus phages, L-shaped tail fibers of coliphage T5, and K5 lyases, the tail spike proteins of phages infecting E. coli K5. By analyzing a representative of each family, we show that in all cases, the CTD is cleaved off after a strictly conserved serine residue and alanine substitution prevented cleavage. Further structural and functional analyses revealed that (i) CTDs are autonomous domains with a high -helical content; (ii) proteolytically released CTDs assemble into hexamers, which are most likely dimers of trimers; (iii) highly conserved amino acids within the CTD are indispensable for CTD-mediated folding and complex formation; (iv) CTDs can be exchanged between proteins of different families; and (v) proteolytic cleavage is essential to stabilize the native protein complex. Data obtained for full-length and proteolytically processed endosialidase variants suggest that release of the CTD increases the unfolding barrier, trapping the mature trimer in a kinetically stable conformation. In summary, we characterize the CTD as a novel C-terminal chaperone domain, which assists folding and assembly of unrelated phage proteins.

Received for publication, October 10, 2006 , and in revised form, November 16, 2006.

^* This work was supported by the Deutsche Forschungsgemeinschaft (DFG) in the framework of DFG Research Unit 548 (Ge801/7-1). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Fig. S1.

¹ To whom correspondence should be addressed. Tel.: 49-511-532-9807; Fax: 49-511-532-3956; E-mail: muehlenhoff.martina{at}mh-hannover.de.

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