A flexible structure which links the flagellar filament to the drive apparatus in the basal body. Absence of the gene leads to absence of the hook protein, lack of the flagellar filament and thus loss of mobility. Approximately wild-type levels of the flagellar subunits are still produced and accumulate mostly in the cytosol. The N-terminal amino acid sequence was from strain CCUG 951; the DNA sequence was from strain CCUG 17874. Disruption and subcellular localization experiments were performed for both strains. The apparent pI for the hook protein was 5.1. Belongs to the flagella basal body rod proteins family.