Visualizing a Circadian Clock Protein

Abstract

Circadian (daily) biological clocks express characteristics that are difficult to explain by known biochemical mechanisms, and will ultimately require characterizing the structures, functions, and interactions of their molecular components. KaiC is an essential circadian protein in cyanobacteria that forms the core of the KaiABC clock protein complex. We report the crystal structure of the KaiC homohexameric complex at 2.8 A resolution. The structure resembles a double doughnut with a central pore that is partially sealed at one end. The crystal structure reveals ATP binding, inter-subunit organization, a scaffold for Kai-protein complex formation, the location of critical KaiC mutations, and evolutionary relationships to other proteins. A key auto-phosphorylation site on KaiC (T432) is identified from the crystal structure, and mutation of this residue abolishes circadian rhythmicity. The crystal structure of KaiC will be essential for understanding this circadian clockwork and for establishing its links to global gene expression.

Roles of Two ATPase-Motif-containing Domains in Cyanobacterial Circadian Clock Protein KaiC

“…strain PCC 7942 KaiC hexamer with 12 molecules of ATP␥S was determined (16). Consistent with our pot-shaped structure of T. elongatus KaiC hexamer determined by single particle analysis (6), the structure was relatively wide and open at the N-terminal end and narrower for a part of the C-terminal domain.…”

“…7). In their structure, ATP binding modes are different between N-terminal and C-terminal domains, and different roles of the two domains were anticipated (16).…”

Identification of key phosphorylation sites in the circadian clock protein KaiC by crystallographic and mutagenetic analyses

“…We recently reported the crystal structure of the full-length core protein from the cyanobacterial circadian clock system, the KaiC homohexamer, at 2.8-Å resolution (10). The structure assumes the shape of a double doughnut with approximate dimensions of 100 by 100 Å (Fig.…”

“…The three-dimensional structures of the proteins encoded by the kai genes have been determined (3,(5)(6)(7)(8)(9)(10). The Kai proteins interact with each other (11,12) to form large complexes in vivo in which KaiC is the core (13).…”

Meshing the gears of the cyanobacterial circadian clock

“…With fitting élan, a small community of scientists has ripped open the packaging of the cyanobacterial circadian clock, compiled the parts list, examined the gears, and begun to piece together the mechanism. Over the past 2 years, the 3D molecular structures have been solved for the core components of the cyanobacterial circadian clock: KaiA, KaiB, and KaiC (1)(2)(3)(4)(5)(6). In a surprisingly literal analogy to mechanical timepieces, the protein that seems to be at the heart of the clock mechanism, KaiC, forms a hexameric ring that even looks like a cog: the escape wheel, perhaps (5,7,8).…”