An interaction of heart disease-associated proteins POPDC1/2 with XIRP1 in transverse tubules and intercalated discs

Abstract

Background Popeye domain-containing proteins 1 and 2 (POPDC1 and POPDC2) are transmembrane proteins involved in cyclic AMP-mediated signalling processes and are required for normal cardiac pacemaking and conduction. In order to identify novel protein interaction partners, POPDC1 and 2 proteins were attached to beads and compared by proteomic analysis with control beads in the pull-down of proteins from cultured human skeletal myotubes. Results There were highly-significant interactions of both POPDC1 and POPDC2 with XIRP1 (Xin actin binding repeat-containing protein 1), actin and, to a lesser degree, annexin A5. In adult human skeletal muscle, both XIRP1 and POPDC1/2 were present at the sarcolemma and in T-tubules. The interaction of POPDC1 with XIRP1 was confirmed in adult rat heart extracts. Using new monoclonal antibodies specific for POPDC1 and POPDC2, both proteins, together with XIRP1, were found mainly at intercalated discs but also at T-tubules in adult rat and human heart. Conclusions Mutations in human POPDC1, POPDC2 and in human XIRP1, all cause pathological cardiac arrhythmias, suggesting a possible role for POPDC1/2 and XIRP1 interaction in normal cardiac conduction.

The Role of POPDC Proteins in Cardiac Pacemaking and Conduction

“…Nevertheless, whether POPDC modulation of XIRP1 function is controlled via cAMP signaling is currently unknown. The same study that reported the XIRP1 interaction also demonstrated an interaction of POPDC1 with annexin A5 (ANXA5) [46]. Together with other annexin subunits, ANXA5 is recruited to the injured plasma membrane and forms a 2-dimensional array preventing an enlargement of the damaged area [128].…”

“…Apart from some alterations in the structure of the sinus node cells, no structural defect has been described in POPDC mutants. However, POPDC proteins have a prominent expression in the intercalated disc (ID) and also interact with several proteins such as ZO1 [43] or XIRP1 [46], which suggests that they might possibly also have a role in controlling structure and function of the ID. From these concluding remarks, it becomes apparent that despite all the progress made and knowledge accumulated in the past 20 years, still more work on POPDC proteins needs to be conducted to better define their molecular functions in the heart and beyond.…”

“…Proteomic analysis revealed that all three POPDC proteins have β-adrenergic receptor (βAR)-dependent phosphorylation sites within their carboxyl terminus [41]. Therefore, β-adrenergic signaling might modulate the biological activity of POPDC proteins not only through cAMP binding, but possibly also through phosphorylation, which could also have an impact on protein-protein interaction [29,[42][43][44][45][46].…”

“…It is therefore possible that POPDC proteins, being an interaction partner of Cav3, are also part of this signaling complex. In ventricular cardiac myocytes of rat, mouse, human, and canine origin, both POPDC1 and POPDC2 are found at the intercalated disk (ID) [46,123]. The ID is a specialized subcellular region of cardiac myocytes and important for their electrical and mechanical coupling [124].…”

“…Three different macromolecular complexes are found in IDs: the fascia adherens which function as actin anchoring sites; gap junctions, which are involved in electrical coupling; and desmosomes and adherens junctions, which are important for mechanical connection of cardiac myocytes [124]. Recently, an interaction of XIRP1 and POPDC1 and POPDC2 has been reported [46]. XIRP1 is a scaffolding protein which modulates the integrity of the ID.…”