title: High affinity dimerization by Ski involves parallel pairing of a novel bipartite alpha-helical domain.
abstract: c-Ski protein possesses a C-terminal dimerization domain that was deleted during the generation of v-ski, and has been implicated in the increased potency of c-ski in cellular transformation compared with the viral gene. The domain is predicted to consist of an extended alpha-helical segment made up of two motifs: a tandem repeat (TR) consisting of five imperfect repeats of 25 residues each and a leucine zipper (LZ) consisting of six heptad repeats. We have examined the structure and dimerization of TR or LZ individually or the entire TR-LZ domain. Using a quenched chemical cross-linking method, we show that the TR dimerizes with moderate efficiency (Kd = 4 x 10(-6) M), whereas LZ dimerizes poorly (Kd > 2 x 10(-5) M). However, the entire TR-LZ domain dimerizes efficiently (Kd = 2 x 10(-8) M), showing a cooperative effect of the two motifs. CD analyses indicate that all three proteins contain predominantly alpha-helices. Limited proteolysis of the TR-LZ dimer indicates that the two helical motifs are linked by a small loop. Interchain disulfide bond formation indicates that both the LZ and TR helices are oriented in parallel. We propose a model for the dimer interface in the TR region consisting of discontinuous clusters of hydrophobic residues forming "leucine buttons."
authors.affiliation: Department of Biochemistry, Case Western Reserve University, School of Medicine, Cleveland, Ohio 44106-4935, USA.
authors.forename: G
authors.initials: G
authors.lastname: Zheng
authors.forename: K M
authors.initials: KM
authors.lastname: Blumenthal
authors.forename: Y
authors.initials: Y
authors.lastname: Ji
authors.forename: D L
authors.initials: DL
authors.lastname: Shardy
authors.forename: S B
authors.initials: SB
authors.lastname: Cohen
authors.forename: E
authors.initials: E
authors.lastname: Stavnezer
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002645:Chickens; D002942:Circular Dichroism; D004268:DNA-Binding Proteins / Q000737:chemistry*; D019281:Dimerization; D008969:Molecular Sequence Data; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D011518:Proto-Oncogene Proteins / Q000737:chemistry*
chemical_list: D004268:DNA-Binding Proteins; D011518:Proto-Oncogene Proteins