Abstract: Series 112, Lecture 3
Lecture #3: Thursday, January 19, 2017 — Time and Location
PKA: Dynamic Assembly of Localized Signaling Complexes
Susan Taylor, PhD
Distinguished Professor, Department of Chemistry & Biochemistry
Distinguished Professor, Department of Pharmacology
University of California, San Diego
La Jolla, California
The PKA catalytic (C) subunit was the first protein kinase to be crystallized and gave us our first glimpse of the fold that describes all protein kinases, which represent one of the largest gene families. The two lobes that comprise the core are flanked by N-terminal and C-terminal tails that wrap around both lobes. By capturing all stages of catalysis in crystal lattices, we appreciate the dynamic nature of these tails and how they undergo order-disorder transitions as part of the catalytic cycle. The activation of each kinase is mediated by the assembly of a hydrophobic “spine” which is part of a finely-tuned hydrophobic core architecture. Assembly of the R-spine is often facilitated by the addition of a critical phosphate, which is another fundamental order/disorder transition that is a conserved feature of most protein kinases. These dynamic features allow the kinases to function as highly regulated molecular switches.
In the case of PKA, the assembled C-subunit is packaged with regulatory (R) subunit dimers where each chain contains two C-terminal cAMP binding (CNB) domains, and there are four functionally non-redundant R-subunits. Activation of each R2C2 holoenzyme is thus dependent on the cAMP second messenger. The CNB domains are highly dynamic allosteric signaling modules that have been conserved across all of biology to translate a biological response to cAMP. Although the fundamental features of this domain were elucidated by structures of isolated R-subunits, it is only the full-length R2C2 holoenzymes of each isoform that allow one to appreciate the importance of symmetry and allostery for PKA signaling. Each R-subunit contains at its N-terminus a dimerization domain that is also a docking site for scaffold proteins referred to as A Kinase Anchoring Proteins (AKAPs). This domain is joined by a flexible linker to the two CNB domains, and embedded within the linker is an inhibitor site that docks to the active site of the C-subunit in the holoenzyme. The order/disorder transitions of the linker regions drive the assembly of the holoenzymes which each have distinct quaternary structures. These dynamic signaling complexes are highly regulated, and a variety of diseases, including cancers and neuropathies, result from mutations that lead to dysfunctional PKA signaling.