title: Oligomerization-dependent association of the SAM domains from Schizosaccharomyces pombe Byr2 and Ste4.
abstract: SAM (sterile alpha motif) domains are protein-protein interaction modules found in a large number of regulatory proteins. Byr2 and Ste4 are two SAM domain-containing proteins in the mating pheromone response pathway of the fission yeast, Schizosaccharomyces pombe. Byr2 is a mitogen-activated protein kinase kinase kinase that is regulated by Ste4. Tu et al. (Tu, H., Barr, M., Dong, D. L., and Wigler, M. (1997) Mol. Cell. Biol. 17, 5876-5887) showed that the isolated SAM domain of Byr2 binds a fragment of Ste4 that contains both a leucine zipper (Ste4-LZ) domain as well as a SAM domain, suggesting that Byr2-SAM and Ste4-SAM may form a hetero-oligomer. Here, we show that the individual SAM domains of Ste4 and Byr2 are monomeric at low concentrations and bind to each other in a 1:1 stoichiometry with a relatively weak dissociation constant of 56 +/- 3 microm. Inclusion of the Ste4-LZ domain, which determines the oligomeric state of Ste4, has a dramatic effect on binding affinity, however. We find that the Ste4-LZ domain is trimeric and, when included with the Ste4-SAM domain, yields a 3:1 Ste4-LZ-SAM:Byr2-SAM complex with a tight dissociation constant of 19 +/- 4 nm. These results suggest that the Ste4-LZ-SAM protein may recognize multiple binding sites on Byr2-SAM, indicating a new mode of oligomeric organization for SAM domains. The fact that high affinity binding occurs only with the addition of an oligomerization domain suggests that it may be necessary to include ancillary oligomerization modules when searching for binding partners of SAM domains.
authors.affiliation: Department of Chemistry and Biochemistry, Molecular Biology Institute, and the UCLA-DOE Laboratory of Structural Biology and Molecular Medicine, University of California, Los Angeles, California 90095, USA.
authors.forename: Ranjini
authors.initials: R
authors.lastname: Ramachander
authors.forename: Chongwoo A
authors.initials: CA
authors.lastname: Kim
authors.forename: Martin L
authors.initials: ML
authors.lastname: Phillips
authors.forename: Cameron D
authors.initials: CD
authors.lastname: Mackereth
authors.forename: Christopher D
authors.initials: CD
authors.lastname: Thanos
authors.forename: Lawrence P
authors.initials: LP
authors.lastname: McIntosh
authors.forename: James U
authors.initials: JU
authors.lastname: Bowie
mesh_terms: D001665:Binding Sites; D002850:Chromatography, Gel; D019281:Dimerization; D004305:Dose-Response Relationship, Drug; D005656:Fungal Proteins / Q000737:chemistry*; D044387:GTP-Binding Protein beta Subunits; D005982:Glutathione Transferase / Q000378:metabolism; D020962:Heterotrimeric GTP-Binding Proteins / Q000737:chemistry*; D020930:MAP Kinase Kinase Kinases; D020928:Mitogen-Activated Protein Kinases / Q000737:chemistry*; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins; D012568:Schizosaccharomyces / Q000201:enzymology*; D029702:Schizosaccharomyces pombe Proteins; D020349:Surface Plasmon Resonance
chemical_list: D005656:Fungal Proteins; D044387:GTP-Binding Protein beta Subunits; D011993:Recombinant Fusion Proteins; D029701:Saccharomyces cerevisiae Proteins; D029702:Schizosaccharomyces pombe Proteins; C067576:Ste4 protein, S cerevisiae; D005982:Glutathione Transferase; D020928:Mitogen-Activated Protein Kinases; C077892:BYR2 protein, S pombe; D020930:MAP Kinase Kinase Kinases; D020962:Heterotrimeric GTP-Binding Proteins