The bacterial flagellar cap as the rotary promoter of flagellin self-assembly

The growth of the bacterial flagellar filament occurs at its distal end by self-assembly of flagellin transported from the cytoplasm through the narro w central channel. The cap at the growing end is essential for its growth, remaining stably attached while permitting the flagellin insertion. In orde r to understand the assembly mechanism, we used electron microscopy to stud y the structures of the cap-filament complex and isolated cap dimer. Five l og-like anchor domains of the pentameric cap flexibly adjusted their confor mations to keep just one flagellin binding site open, indicating a cap rota tion mechanism to promote the flagellin self-assembly. This represents one of the most dynamic movements in protein structures.