Periostin and TGF-β-induced protein: Two peas in a pod?

Periostin (PN) and TGF-β-induced protein (βig-h3) are paralogs that contain a single emilin and four fasciclin-1 modules and are secreted from cells. PN receives attention because of its up-regulation in cancer and degenerative and allergic diseases. βig-h3 is highly enriched in cornea and best known for harboring mutations in humans associated with corneal dystrophies. Both proteins are expressed widely, and many functions, some over-lapping, have been attributed to PN and βig-h3 based on biochemical, cell culture, and whole animal experiments. We attempt to organize this knowledge so as to facilitate research on these interesting and incompletely understood proteins. We focus particularly on whether PN and βig-h3 are modified by vitamin K-dependent γ-glutamyl carboxylation, a question of considerable importance given the profound effects of γ-carboxylation on structure and function of other proteins. We consider the roles of PN and βig-h3 in formation of extracellular matrix and as ligands for integrin receptors. We attempt to reconcile the contradictory results that have arisen concerning the role of PN, which has emerged as a marker of TH2 immunity, in murine models of allergic asthma. Finally, when possible we compare and contrast the structures and functions of the two proteins.

The beta-adrenergic receptor kinase (GRK2) is regulated by phospholipids

The beta-adrenergic receptor kinase (beta ARK) is a member of growing family of G protein coupled receptor kinases (GRKs). beta ARK and other members of the GRK family play a role in the mechanism of agonist-specific desensitization by virtue of their ability to phosphorylate G protein-coupled receptors in an agonist-dependent manner. beta ARK activation is known to occur following the interaction of the kinase with the agonist-occupied form of the receptor substrate and heterotrimeric G protein beta gamma subunits. Recently, lipid regulation of GRK2, GRK3, and GRK5 have also been described. Using a mixed micelle assay, GRK2 (beta ARK1) was found to require phospholipid in order to phosphorylate the beta 2-adrenergic receptor. As determined with a nonreceptor peptide substrate of beta ARK, catalytic activity of the kinase increased in the presence of phospholipid without a change in the Km for the peptide. Data obtained with the heterobifunctional cross-linking agent N-3-[125I]iodo-4-azidophenylpropionamido-S-(2-thiopyridyl)-c ysteine ([125I]ACTP) suggests that the activation by phospholipid was associated with a conformational change in the kinase. [125I]ACTP incorporation increased 2-fold in the presence of crude phosphatidylcholine, and this increase in [125I]ACTP labeling is completely blocked by the addition of MgATP. Furthermore, proteolytic mapping was consistent with the modification of a distinct site when GRK2 was labeled in the presence of phospholipid. While an acidic phospholipid specificity was demonstrated using the mixed micelle phosphorylation assay, a notable exception was observed with PIP2. In the presence of PIP2, kinase activity as well as [125I]ACTP labeling was inhibited. These data demonstrate the direct regulation of GRK2 activity by phospholipids and supports the hypothesis that this effect is the result of a conformational change within the kinase.