ROLE OF THE OUTERMOST SUBDOMAIN OF SALMONELLA FLAGELLIN IN THE FILAMENT STRUCTURE REVEALED BY ELECTRON CRYOMICROSCOPY

A mutant strain of Salmonella typhimurium, SJW46, has flagellar filame nts supercoiled in the same form as the wild-type strain, SJW1103, and swims normally. However, its flagellar filaments are mechanically uns table and show anomalous behaviors of polymorphism. Flagellin from SJW 46 has a large central deletion from Ala204 to Lys292 of SJW1103 flage llin, which has been thought to be located in the outer surface of the filament. Since the filament structure is determined by intersubunit interactions of the terminal regions in the densely packed core of the filament, no serious involvement of the deleted portion was expected in the filament stability and polymorphism. In order to locate the del eted portion and to understand the underlying mechanism of these anoma lous characteristics, we carried out structure analysis of the L-type straight filament reconstituted from a mutant flagellin of SJW46 (SJW4 6S) and compared the structure with that of the SJW1660 filament, whic h is also the L-type but composed of flagellin with no deletion. The d eleted portion was identified as the outermost subdomain, and the stru cture in the core region showed no appreciable differences. The struct ure revealed the previously identified folding of flagellin in further detail, and the significance of intersubunit interactions between out er domains, which are present in the SJW1660 filament but absent in th e SJW46 filament. This suggests that these contacts have a significant contribution to the filament stability and polymorphic behavior, desp ite the fact that the contacting surface area occupies only a minor po rtion of the whole intersubunit interactions. (C) 1998 Academic Press.