Classical A kinase anchor proteins (AKAPs) preferentially tether type II protein kinase A (PKAII) isoforms to sites in the cytoskeleton and organelles. It is not known if distinct proteins selectively sequester regulatory (R) subunits of type I PKAs, thereby diversifying functions of these critical enzymes. In Caenorhabditis elegans, a single type I PKA mediates all aspects of cAMP signaling. We have discovered a cDNA that encodes a binding protein (AKAP CE ) for the regulatory subunit (R CE ) of C. elegans PKAI CE . AKAP CE is a novel, highly acidic RING finger protein composed of 1,280 amino acids. It binds RI-like R CE with high affinity and neither RII␣ nor RII competitively inhibits formation of AKAP CE ⅐R CE complexes. The R CE -binding site was mapped to a segment of 20 amino acids in an N-terminal region of AKAP CE . Several hydrophobic residues in the binding site align with essential Leu and Ile residues in the RII-selective tethering domain of prototypic mammalian AKAPs. However, the R CE -binding region in AKAP CE diverges sharply from consensus RII-binding sites by inclusion of three aromatic amino acids, exclusion of a highly conserved Leu or Ile at position 8 and replacement of Cterminal hydrophobic amino acids with basic residues. AKAP CE ⅐R CE complexes accumulate in intact cells.A kinase anchor proteins (AKAPs) 1 avidly bind regulatory subunits (RII and RII␣) of the type II isoforms of cAMP-dependent protein kinase (PKAII and PKAII␣) (1-3). The various AKAPs are functional and not structural homologs; they possess partly homologous RII-binding domains, but the remaining portions of their sequences are divergent. Moreover, different AKAPs target PKAII isoforms to distinct intracellular loci including mitochondria, peroxisomes, Golgi, and plasma membranes and sites in cytoskeleton (1-6). This accomplishes incorporation of AKAP⅐PKAII complexes into signaling modules that are either juxtaposed with or spatially segregated from hormone-activated adenylate cyclase. Substrate-effector proteins are thought to be clustered in proximity with anchored PKAII.RI␣ is expressed in most tissues, where it mediates (as PKAI␣) actions of many hormones. It is the only R isoform that is essential for the progression of embryogenesis (7). Persistent synthesis of low levels of free RI␣ serves as a critical homeostatic mechanism to maintain the total R:PKA catalytic subunit (C) ratio at 1 (7). If excess C accumulates it is bound by RI␣, and the t1 ⁄2 of RI␣ increases 5-fold, thereby inhibiting otherwise unregulated and potentially toxic C activity. It is possible that RI␣ (PKAI␣) functions are further diversified by interactions with anchor proteins. This topic received little attention because fractionation of cell and tissue homogenates and immunocytochemical localization analysis indicate that the bulk of PKAI is dispersed in cytoplasm. However, RI is bound to the plasma membrane of human erythrocytes, and PKAI is recruited to a multi-protein complex at the "cap" site of activated T lymphocytes (8, 9). Proteins ...
Caenorhabditis elegans protein kinase A (PKAI CE) (5,6,9,10). Clustering of a high concentration of AKAP⅐PKAII complexes in proximity with PKA substrate/effector proteins in cytoskeleton/organelles enables efficient reception, rapid amplification, and precisely focused targeting of cAMP signals. Consequently, PKA-catalyzed phosphorylation of co-localized effector proteins is optimized (11-13). Key tenets of the preceding signaling model have been verified. Disruption of AKAP⅐PKA complexes in situ markedly diminishes the ability of cAMP to regulate such critical physiological processes as ion transport, gene transcription, apoptosis, and hormone secretion (8,(13)(14)(15)(16).Mammals employ Ͼ20 distinct AKAPs to adapt type II PKAs for specialized functions (5)(6)(7)(8)17). Amino acid sequences of the anchor proteins are markedly divergent; thus, AKAPs are functional, not structural, homologs. Different AKAPs accumulate in distinct locations; examples include cytoplasmic surfaces of plasma membrane, mitochondria, Golgi membranes, and centrosomes (5,6,9,10,18). Routing of PKAII to specific microenvironments is governed by unique targeting/anchoring domains in individual AKAPs (9, 19 -22). Primary structures of RII-binding sites in AKAPs are not highly conserved (e.g. see alignments in Refs. 23 and 24). However, the folding pattern and physical properties of amino acids that subserve the ligation of RII subunits are universal properties of AKAPs. The RII-binding region of typical AKAPs comprises 16 -20 contiguous amino acids that have a predicted propensity to fold into an amphipathic ␣-helix (25). The ␣-helix includes six critical, precisely positioned amino acids whose large aliphatic side chains co-operatively create an extended hydrophobic surface (19). The size, hydrophobicity, and configuration of this domain generate a unique receptor site for a complementary, apolar docking surface that is produced from the folding of ϳ30 amino acid residues in RII␣ or RII dimers (26 -29). Robust hydrophobic * This work was supported by National Institutes of Health Grant GM57660 and a stipend from Training Grant GM07620 (to R. A. A). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.‡ To whom correspondence should be addressed: Dept.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.