Protein kinase A
Protein kinase A (PKA) is a kind of serine/threonine kinase that phosphorylates serine or threonine residue of substrate after the increase of cytoplasmic cAMP concentration. Physiological regulation of PKA is versatile: for example in cardiac myocyte, phosphorylation of plasmamembrane- and endoplasmic proteins by PKA increases intracellular calcium recruitment and enforce contraction.
Fig.1 Scheme of PKA holoenzyme activation
PKA is formed by a holoenzyme, in which catalytic (C) and regulatory (R) subunits associate. The C subunit contains the active center whereas the R subunit has two cAMP binding sites. This C and R subunit complex is inactive state. Binding of cAMP to R subunit induces C subunit activation accompanied by the dissociation of R subunit from C subunit. Recent structural analysis has elucidated the precise molecular basis of PKA activation.
Fig.2 Overall structure of holoenzyme
The C subunit (CIα)－R subunit (RIα(91-379: R333K)) complex is shown here. C subunit has two (large and small) lobes. R subunit has two cAMP binding domains (domain A and domain B) and an αB/C helix.
Fig.3 Dramatic conformational change of R subunit by cAMP binding
The structures of C subunit bound (cAMP free: left) R sununit and cAMP bound R subunit (2RGS: right) and are compared here. The most dramatic difference is the conformation of αB/C helix; αB/C helix is dramatically twisted in cAMP bound R subunit. cAMP binding to domain B of R subunit would disrupt of the salt bridge between R361 in domain B and E261 in αB/C helix, which stabilizes the conformation of αB/C helix.
Fig.4 Model of cAMP mediated PKA activation
The finding shown in fig.3 suggests a PKA activation model as described below:
(1) cAMP binds to domain B.
(2) Salt bridge between domain B and αB/C helix is broken.
(3) αB/C twisting induces dramatic conformational change of R subunit.
(4) cAMP binds to domain A.
(5) R subunit dissociates from C subunit.
Protein Data Bank (PDB)
Kim, C. Cheng, C.Y. Saldanha, S.A. Taylor, S.S.; "PKA-I holoenzyme structure reveals a mechanism for cAMP-dependent activation."; Cell(Cambridge,Mass.); (2007) 130:1032-1043 PubMed:17889648.
author: Daisuke Ino