Identification of C-terminal hydrophobic residues important for dimerization and all known functions of ParB of Pseudomonas aeruginosa

Abstract

The ParB protein of P. aeruginosa is important for growth, cell division, nucleoid segregation and different types of motility. To further understand its function we have demonstrated a vital role of the hydrophobic residues in the C-terminus of ParBP.a. By in silico modeling of the C-terminal domain (242-290 amino acids) the hydrophobic residues L282, V285 and I289 (but not L286) are engaged in leucine zipper-like structure formation, whereas the charged residues R290 and Q266 are implicated in forming a salt bridge involved in protein stabilization. Five parB mutant alleles were constructed and their functionality defined in vivo and in vitro. In agreement with model predictions the substitution of L286A had no effect on mutant protein activities. Two ParBs with single substitutions of L282A or V285A and deletions of two or seven C-terminal amino acids were impaired in both dimerization and DNA binding and were not able to silence genes adjacent to parS suggesting that dimerization through the C-terminus is a prerequisite for spreading on DNA. The defect in dimerization also correlated with loss of ability to interact with partner protein ParA. Reverse genetics demonstrated that a parB mutant producing ParB lacking the two C-terminal amino acids as well as mutants producing ParB with single substitution L282A or V285A had defects comparable to those of a parB null mutant. Thus so far all the properties of ParB seem to depend on dimerization.