title: Dynamic in vivo interactions among Myc network members.
abstract: Members of the Myc oncoprotein network (c-Myc, Max, and Mad) play important roles in proliferation, differentiation, and apoptosis. We expressed chimeric green fluorescent protein (GFP) fusions of c-Myc, Max, and three Mad proteins in fibroblasts. Individually, c-Myc and Mad proteins localized in subnuclear speckles, whereas Max assumed a homogeneous nuclear pattern. These distributions were co-dominant and dynamic, however, as each protein assumed the pattern of its heterodimeric partner when the latter was co-expressed at a higher level. Deletion mapping of two Mad members, Mad1 and Mxi1, demonstrated that the domains responsible for nuclear localization and speckling are separable. A non-speckling Mxi1 mutant was also less effective as a transcriptional repressor than wild-type Mxi1. c-Myc nuclear speckles were distinct from SC-35 domains involved in mRNA processing. However, in the presence of co-expressed Max, c-Myc, but not Mad, co-localized to a subset of SC-35 loci. These results show that Myc network proteins comprise dynamic subnuclear structures and behave co-dominantly when co-expressed with their normal heterodimerization partners. In addition, c-Myc-Max heterodimers, but not Max-Mad heterodimers, localize to foci actively engaged in pre-mRNA transcription/processing. These findings suggest novel means by which Myc network members promote transcriptional activation or repression.
authors.affiliation: Section of Hematology/Oncology, Department of Pediatrics, Children's Hospital of Pittsburgh, Pittsburgh, Pennsylvania, PA 15213, USA.
authors.forename: X
authors.initials: X
authors.lastname: Yin
authors.forename: M F
authors.initials: MF
authors.lastname: Landay
authors.forename: W
authors.initials: W
authors.lastname: Han
authors.forename: E S
authors.initials: ES
authors.lastname: Levitan
authors.forename: S C
authors.initials: SC
authors.lastname: Watkins
authors.forename: R M
authors.initials: RM
authors.lastname: Levenson
authors.forename: D L
authors.initials: DL
authors.lastname: Farkas
authors.forename: E V
authors.initials: EV
authors.lastname: Prochownik
mesh_terms: D016475:3T3 Cells; D000818:Animals; D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D051792:Basic Helix-Loop-Helix Proteins; D050976:Basic-Leucine Zipper Transcription Factors; D015153:Blotting, Western; D019556:COS Cells; D002451:Cell Compartmentation; D018797:Cell Cycle Proteins; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D049452:Green Fluorescent Proteins; D020672:I-kappa B Proteins; D008164:Luminescent Proteins / Q000235:genetics / Q000378:metabolism; D051379:Mice; D000072000:NF-KappaB Inhibitor alpha; D009687:Nuclear Proteins; D010750:Phosphoproteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics / Q000378:metabolism*; D012323:RNA Processing, Post-Transcriptional; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012097:Repressor Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D014157:Transcription Factors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D014158:Transcription, Genetic; D025521:Tumor Suppressor Proteins
chemical_list: D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D051792:Basic Helix-Loop-Helix Proteins; D050976:Basic-Leucine Zipper Transcription Factors; D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; D049452:Green Fluorescent Proteins; D020672:I-kappa B Proteins; D008164:Luminescent Proteins; D000072000:NF-KappaB Inhibitor alpha; D009687:Nuclear Proteins; D010750:Phosphoproteins; D016271:Proto-Oncogene Proteins c-myc; D011993:Recombinant Fusion Proteins; D012097:Repressor Proteins; D014157:Transcription Factors; D025521:Tumor Suppressor Proteins; C079440:MXI1 protein, human; C501385:Mad1l1 protein, mouse; C499329:Max protein, rat; C512788:Mxi1 protein, mouse; C512789:Mxi1 protein, rat; C067850:Myc associated factor X; C000606204:NFKBIA protein, human; C000606205:Nfkbia protein, mouse; C000606206:Nfkbia protein, rat; C068422:Max protein, mouse