Different Routes of Bone Morphogenic Protein (BMP) Receptor Endocytosis Influence BMP Signaling

Abstract

Endocytosis is important for a variety of functions in eukaryotic cells, including the regulation of signaling cascades via transmembrane receptors. The internalization of bone morphogenetic protein (BMP) receptor type I (BRI) and type II (BRII) and its relation to signaling were largely unexplored. Here, we demonstrate that both receptor types undergo constitutive endocytosis via clathrin-coated pits (CCPs) but that only BRII undergoes also caveola-like internalization. Using several complementary approaches, we could show that (i) BMP-2-mediated Smad1/5 phosphorylation occurs at the plasma membrane in nonraft regions, (ii) continuation of Smad signaling resulting in a transcriptional response requires endocytosis via the clathrin-mediated route, and (iii) BMP signaling leading to alkaline phosphatase induction initiates from receptors that fractionate into cholesterol-enriched, detergent-resistant membranes. Furthermore, we show that BRII interacts with Eps15R, a constitutive component of CCPs, and with caveolin-1, the marker protein of caveolae. Taken together, the localization of BMP receptors in distinct membrane domains is prerequisite to their taking different endocytosis routes with specific impacts on Smad-dependent and Smad-independent signaling cascades.

Supplemental material for this article may be found at http://mcb.asm.org/.

We thank Georg Krohne for advice on electron microscopy and Ernst Conzelmann for assistance during lipid chromatography. We are especially grateful to the excellent technical support of Nadine Yurdagül-Hemmrich. We thank Walter Sebald for recombinant BMP-2 and helpful discussions.

This work was supported in part by grants from the DFG (Kn332/8-1 to P.K.), from the Israel Science Foundation (grant 185/05), and from the Israel Cancer Research Fund (to Y.I.H.). Y.I.H. holds the Zalman Weinberg Chair in Cell Biology.