title: SAM domain-based protein oligomerization observed by live-cell fluorescence fluctuation spectroscopy.
abstract: Sterile-alpha-motif (SAM) domains are common protein interaction motifs observed in organisms as diverse as yeast and human. They play a role in protein homo- and hetero-interactions in processes ranging from signal transduction to RNA binding. In addition, mutations in SAM domain and SAM-mediated oligomers have been linked to several diseases. To date, the observation of heterogeneous SAM-mediated oligomers in vivo has been elusive, which represents a common challenge in dissecting cellular biochemistry in live-cell systems. In this study, we report the oligomerization and binding stoichiometry of high-order, multi-component complexes of (SAM) domain proteins Ste11 and Ste50 in live yeast cells using fluorescence fluctuation methods. Fluorescence cross-correlation spectroscopy (FCCS) and 1-dimensional photon counting histogram (1dPCH) confirm the SAM-mediated interaction and oligomerization of Ste11 and Ste50. Two-dimensional PCH (2dPCH), with endogenously expressed proteins tagged with GFP or mCherry, uniquely indicates that Ste11 and Ste50 form a heterogeneous complex in the yeast cytosol comprised of a dimer of Ste11 and a monomer of Ste50. In addition, Ste50 also exists as a high order oligomer that does not interact with Ste11, and the size of this oligomer decreases in response to signals that activate the MAP kinase cascade. Surprisingly, a SAM domain mutant of Ste50 disrupted not only the Ste50 oligomers but also Ste11 dimerization. These results establish an in vivo model of Ste50 and Ste11 homo- and hetero-oligomerization and highlight the usefulness of 2dPCH for quantitative dissection of complex molecular interactions in genetic model organisms such as yeast.
authors.affiliation: The Stowers Institute for Medical Research, Kansas City, Missouri, United States of America.
authors.forename: Brian D
authors.initials: BD
authors.lastname: Slaughter
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Huff
authors.forename: Winfried
authors.initials: W
authors.lastname: Wiegraebe
authors.forename: Joel W
authors.initials: JW
authors.lastname: Schwartz
authors.forename: Rong
authors.initials: R
authors.lastname: Li
mesh_terms: D004058:Diffusion; D049452:Green Fluorescent Proteins / Q000378:metabolism; D020930:MAP Kinase Kinase Kinases / Q000737:chemistry / Q000378:metabolism; D050296:Microbial Viability; D017785:Photons; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000166:cytology* / Q000201:enzymology; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry* / Q000378:metabolism; D013050:Spectrometry, Fluorescence / Q000379:methods*
chemical_list: D011993:Recombinant Fusion Proteins; C079506:STE50 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D049452:Green Fluorescent Proteins; D020930:MAP Kinase Kinase Kinases; C067018:Ste11 protein, S cerevisiae