title: The four canonical tpr subunits of human APC/C form related homo-dimeric structures and stack in parallel to form a TPR suprahelix.
abstract: The anaphase-promoting complex or cyclosome (APC/C) is a large E3 RING-cullin ubiquitin ligase composed of between 14 and 15 individual proteins. A striking feature of the APC/C is that only four proteins are involved in directly recognizing target proteins and catalyzing the assembly of a polyubiquitin chain. All other subunits, which account for >80% of the mass of the APC/C, provide scaffolding functions. A major proportion of these scaffolding subunits are structurally related. In metazoans, there are four canonical tetratricopeptide repeat (TPR) proteins that form homo-dimers (Apc3/Cdc27, Apc6/Cdc16, Apc7 and Apc8/Cdc23). Here, we describe the crystal structure of the N-terminal homo-dimerization domain of Schizosaccharomyces pombe Cdc23 (Cdc23(Nterm)). Cdc23(Nterm) is composed of seven contiguous TPR motifs that self-associate through a related mechanism to those of Cdc16 and Cdc27. Using the Cdc23(Nterm) structure, we generated a model of full-length Cdc23. The resultant "V"-shaped molecule docks into the Cdc23-assigned density of the human APC/C structure determined using negative stain electron microscopy (EM). Based on sequence conservation, we propose that Apc7 forms a homo-dimeric structure equivalent to those of Cdc16, Cdc23 and Cdc27. The model is consistent with the Apc7-assigned density of the human APC/C EM structure. The four canonical homo-dimeric TPR proteins of human APC/C stack in parallel on one side of the complex. Remarkably, the uniform relative packing of neighboring TPR proteins generates a novel left-handed suprahelical TPR assembly. This finding has implications for understanding the assembly of other TPR-containing multimeric complexes.
authors.affiliation: Division of Structural Biology, Institute of Cancer Research, Chester Beatty Laboratories, 237 Fulham Road, London SW3 6JB, UK.
authors.forename: Ziguo
authors.initials: Z
authors.lastname: Zhang
authors.forename: Leifu
authors.initials: L
authors.lastname: Chang
authors.forename: Jing
authors.initials: J
authors.lastname: Yang
authors.forename: Nora
authors.initials: N
authors.lastname: Conin
authors.forename: Kiran
authors.initials: K
authors.lastname: Kulkarni
authors.forename: David
authors.initials: D
authors.lastname: Barford
mesh_terms: D000595:Amino Acid Sequence; D064173:Anaphase-Promoting Complex-Cyclosome / Q000737:chemistry* / Q000648:ultrastructure; D064191:Apc3 Subunit, Anaphase-Promoting Complex-Cyclosome / Q000737:chemistry / Q000378:metabolism; D064194:Apc6 Subunit, Anaphase-Promoting Complex-Cyclosome / Q000737:chemistry / Q000378:metabolism; D064195:Apc7 Subunit, Anaphase-Promoting Complex-Cyclosome / Q000737:chemistry; D018797:Cell Cycle Proteins / Q000737:chemistry; D006801:Humans; D064110:Minichromosome Maintenance Proteins / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D054730:Protein Interaction Domains and Motifs; D055503:Protein Multimerization; D021122:Protein Subunits; D012441:Saccharomyces cerevisiae / Q000737:chemistry / Q000378:metabolism; D012568:Schizosaccharomyces / Q000737:chemistry / Q000378:metabolism; D029702:Schizosaccharomyces pombe Proteins / Q000737:chemistry; D016415:Sequence Alignment
chemical_list: D064191:Apc3 Subunit, Anaphase-Promoting Complex-Cyclosome; D064194:Apc6 Subunit, Anaphase-Promoting Complex-Cyclosome; D064195:Apc7 Subunit, Anaphase-Promoting Complex-Cyclosome; D018797:Cell Cycle Proteins; D021122:Protein Subunits; D029702:Schizosaccharomyces pombe Proteins; C489609:cdc23 protein, S pombe; D064173:Anaphase-Promoting Complex-Cyclosome; D064110:Minichromosome Maintenance Proteins
keywords: APC/C; EM; FOM; PDB; Protein Data Bank; SAD; TPR; anaphase-promoting complex; anaphase-promoting complex or cyclosome; cell cycle; crystallography; electron microscopy; figure of merit; single-particle electron microscopy; single-wavelength anomalous dispersion; tetratricopeptide repeat; tetratricopeptide repeat (TPR)