A Direct Role for ATP1A1 in Unconventional Secretion of Fibroblast Growth Factor 2

Sonja Zacherl,Giuseppe La Venuta,Hans-Michael Müller,Sabine Wegehingel,Eleni Dimou,Peter Sehr,Joe D Lewis,Holger Erfle,Rainer Pepperkok,Walter Nickel

doi:10.1074/jbc.m114.590067

Abstract

Previous studies proposed a role for the Na/K-ATPase in unconventional secretion of fibroblast growth factor 2 (FGF2). This conclusion was based upon pharmacological inhibition of FGF2 secretion in the presence of ouabain. However, neither independent experimental evidence nor a potential mechanism was provided. Based upon an unbiased RNAi screen, we now report the identification of ATP1A1, the α1-chain of the Na/K-ATPase, as a factor required for efficient secretion of FGF2. As opposed to ATP1A1, down-regulation of the β1- and β3-chains (ATP1B1 and ATP1B3) of the Na/K-ATPase did not affect FGF2 secretion, suggesting that they are dispensable for this process. These findings indicate that it is not the membrane potential-generating function of the Na/K-ATPase complex but rather a so far unidentified role of potentially unassembled α1-chains that is critical for unconventional secretion of FGF2. Consistently, in the absence of β-chains, we found a direct interaction between the cytoplasmic domain of ATP1A1 and FGF2 with submicromolar affinity. Based upon these observations, we propose that ATP1A1 is a recruitment factor for FGF2 at the inner leaflet of plasma membranes that may control phosphatidylinositol 4,5-bisphosphate-dependent membrane translocation as part of the unconventional secretory pathway of FGF2.

Highlights

Unconventional secretion of fibroblast growth factor 2 (FGF2) occurs by direct translocation across plasma membranes
We have shown that this pathway is initiated by recruitment of FGF2 to the inner leaflet of plasma membranes mediated by the phosphoinositide phosphatidylinositol 4,5bisphosphate [14, 15]
In the current study, based on an unbiased large scale RNAi screen through which Tec kinase was identified as a positive regulator of FGF2 secretion [21], we report on the identification of another component of the FGF2 secretion machinery, ATP1A1, the ␣1-chain of the Na/K-ATPase

Summary

Introduction

Unconventional secretion of FGF2 occurs by direct translocation across plasma membranes. As opposed to ATP1A1, down-regulation of the ␤1- and ␤3-chains (ATP1B1 and ATP1B3) of the Na/K-ATPase did not affect FGF2 secretion, suggesting that they are dispensable for this process These findings indicate that it is not the membrane potentialgenerating function of the Na/K-ATPase complex but rather a so far unidentified role of potentially unassembled ␣1-chains that is critical for unconventional secretion of FGF2. In the absence of ␤-chains, we found a direct interaction between the cytoplasmic domain of ATP1A1 and FGF2 with submicromolar affinity Based upon these observations, we propose that ATP1A1 is a recruitment factor for FGF2 at the inner leaflet of plasma membranes that may control phosphatidylinositol 4,5-bisphosphate-dependent membrane translocation as part of the unconventional secretory pathway of FGF2

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Conclusion