The Fos wild-type leucine zipper is unable to support homodimerization. This finding is generally explained by the negative net charge of the Fos zipper leading to the electrostatic repulsion of two monomers. Using a LexA-dependent in vivo assay in Escherichia coli, we show here that additional antideterminants for Fos zipper association are the residues in position a within the Fos zipper interface. If the wild-type Fos zipper is fused to the DNA binding domain of the LexA repressor (LexA-DBD), no excess repression is observed as compared with the LexA-DBD alone, in agreement with the incapacity of the wild-type Fos zipper to promote homodimerization. If hydrophobic amino acids (Ile, Leu, Val, Phe, Met) are inserted into the five a positions of a LexA-Fos zipper fusion protein, substantial transcriptional repression is recovered showing that Fos zipper homodimerization is not only limited by the repulsion of negatively charged residues but also by the nonhydrophobic nature of the a positions. The most efficient variants (harboring Ile or Leu in the five a positions) show an about 80-fold increase in transcriptional repression as compared with the wild-type Fos zipper fusion protein. In the case of multiple identical substitutions, the overall improvement is correlated with the hydrophobicity of the inserted side chains, i.e. Ile Leu > Val > Phe > Met. However at least for Val, Phe, and Met the impact of a given residue type on the association efficiency depends strongly on the heptad, i.e. on the local environment of the a residue. This is particularly striking for the second heptad of the Fos zipper, where Val is less well tolerated than Phe and Met. Most likely the a residue modulates the interhelical repulsion between two glutamic acid side chains in positions g and e. Most of the hydrophobic Fos zipper variants are also improved in heteroassociation with a Jun leucine zipper, such that roughly half of the additional free energy of homodimerization is imported into the heterodimer. A few candidates (including the Fos wild-type zipper) deviate from this correlation, showing considerable excess heteroassociation.

CiteULike

Complore

Connotea

Del.icio.us

Digg

Reddit

Technorati

This article has been cited by other articles:

				P. Clarke, Pár. Cuív, and M. O'Connell Novel mobilizable prokaryotic two-hybrid system vectors for high-throughput protein interaction mapping in Escherichia coli by bacterial conjugation Nucleic Acids Res., February 1, 2005; 33(2): e18 - e18. [Abstract] [Full Text] [PDF]

				D. Nehme, X.-Z. Li, R. Elliot, and K. Poole Assembly of the MexAB-OprM Multidrug Efflux System of Pseudomonas aeruginosa: Identification and Characterization of Mutations in mexA Compromising MexA Multimerization and Interaction with MexB J. Bacteriol., May 15, 2004; 186(10): 2973 - 2983. [Abstract] [Full Text] [PDF]

				D. Schneider and D. M. Engelman GALLEX, a Measurement of Heterologous Association of Transmembrane Helices in a Biological Membrane J. Biol. Chem., January 24, 2003; 278(5): 3105 - 3111. [Abstract] [Full Text] [PDF]

				H. Watari, E. J. Blanchette-Mackie, N. K. Dwyer, M. Watari, E. B. Neufeld, S. Patel, P. G. Pentchev, and J. F. Strauss III Mutations in the Leucine Zipper Motif and Sterol-sensing Domain Inactivate the Niemann-Pick C1 Glycoprotein J. Biol. Chem., July 30, 1999; 274(31): 21861 - 21866. [Abstract] [Full Text] [PDF]

				Y. Shi, S. Wang, S. Krueger, and F. P. Schwarz Effect of Mutations at the Monomer-Monomer Interface of cAMP Receptor Protein on Specific DNA Binding J. Biol. Chem., March 12, 1999; 274(11): 6946 - 6956. [Abstract] [Full Text] [PDF]