Fifty Years Since the Discovery of PKA

Abstract

In 1968, after Sutherland's discovery of cAMP, Krebs discovered cAMP‐dependent Protein Kinase (PKA), thus establishing the major paradigm for cAMP signaling in mammalian cells. The protein kinases, one of the largest gene families, are associated with many diseases, and PKA serves as a prototype for general protein kinase structure and function. While the structure of PKA first defined the conserved protein kinase fold, the recent solution of inhibited complexes of the catalytic (C) and regulatory (R) subunits have defined not only the molecular basis for inhibition but also the mechanisms for activation by cAMP. In addition, they show the dramatic flexibility that is embedded within the R‐subunit as it releases cAMP and wraps itself around the large lobe of the C‐subunit. In contrast, the C‐subunit functions as a stable scaffold that binds to many proteins. In addition to the R‐subunit isoforms and PKI, the C‐subunit docks to other proteins such as the A Kinase Interacting Protein (AKIP‐1), which contributes to its nuclear localization. The R‐ and C‐subunits do not typically exist as isolated entities in the cell but instead are part of large complexes assembled in part through A Kinase Anchoring Proteins (AKAPs). Localization and trafficking are an essential part of kinase function, and the challenge for the next decade will be to understand how these large PKA‐mediated signaling complexes assemble and integrate highly regulated functional responses. It is a challenge that requires the concerted integration of structural biology, cell biology, pharmacology and physiology. (Funded by grants from the NIH to SST and fellowships from the American Cancer Society to CK and CWE, and NIH Training Grant GM08326 to CYC.)

Auto-regulation of Secretory Flux by Sensing and Responding to the Folded Cargo Protein Load in the Endoplasmic Reticulum

“…To study the molecular make-up of the signaling pathways activated by cargo, given the robust and general role of PKA in ER export, we considered that AKAP (A kinase-anchoring protein) scaffold-based signaling complexes may provide a suitable model. AKAPs are scaffolds that anchor PKA-dependent pathways and other regulatory proteins onto specific cellular structures, often forming large signaling complexes (Scott et al, 2013;Taylor et al, 2008;Wong and Scott, 2004). To elucidate the specific components and organization of the AKAP-based AREX signaling system, we used a series of concatenated folding pulse experiments.…”

“…Cargo Folding Activates and Recruits PKA at the ERESs PKA is comprised of homo-dimers of catalytic and regulatory subunits. The latter bind to a conserved motif in the AKAPs and are activated upon binding cAMP (Carr et al, 1992;Scott et al, 2013), which is followed by detachment of the catalytic subunits (Cancino et al, 2014;Taylor et al, 2008). We found that, in cells kept at 40 C to accumulate unfolded cargo, PKA does not reside at the ERESs; the regulatory subunit PKA-RIIa localizes mainly in an uncharacterized cluster of perinuclear puncta that does not co-stain for the ERES marker Sec31A (Figure 4A,left) and the catalytic subunit PKA-Ca shows a diffuse distribution ( Figure S3A, left).…”