FC0108
Bd0108  -  Bd0109

Biological function
Bd0108 is a central regulatory element to switch from the host-independent (HI) to host-dependent (HD) lifestyle of the bacteria. The proteins Bd0108 and Bd0109 interact directly and work in concert to both promote the secretion of the B. bacteriovorus pilus and to regulate pilus length. Mutations or deletions in Bd0108 lock the B. bacteriovorus in HI form.

Domain organization/sequence features
GSHMADENANRPVNPGEDPN EAFRSTPFEATTSALGDCRE CIAYRTGATTGKGSRRHDDT VSREIKGSSATPGGSEKAGT GRQ

Structural evidence
Bd0108 has no regular secondary structure and functions as a monomeric extended conformational ensemble. In the ¹H-¹⁵N HSQC spectrum the C terminal 74–85 residues of Bd0108 show only moderate signal loss, while residues 89–101 are unaffected by interactions with Bd0109. N-terminal residues 24–66 are the site of binding to Bd0109, which enter into chemical exchange in the presence of Bd0109, on a μsec to msec time scale, also likely adopting many different states/conformations upon interactions.

Biochemical evidence
ITC provides a dissociation constant of 45 μM with an observed stoichiometry of 3:1 (Bd0108:Bd0109). Enthalpy of binding is negative (22075 kcal/mole), the entropy is positive (12.92 cal K^-1 mol^-1), showing that both hydrogen bonding and hydrophobic interactions contribute to binding. The high micro-molar K_d is characteristic of transient interactions, such as those involved in signaling or secretion pathways.

Mechanism category
tethering

Significance
Fuzziness may contribute to interactions with many other binding partners that affect the lifestyle switch of the bacteria.