Sorting Nexin 6, a Novel SNX, Interacts with the Transforming Growth Factor-β Family of Receptor Serine-Threonine Kinases

Abstract

Sorting nexins (SNX) comprise a family of proteins with homology to several yeast proteins, including Vps5p and Mvp1p, that are required for the sorting of proteins to the yeast vacuole. Human SNX1, -2, and -4 have been proposed to play a role in receptor trafficking and have been shown to bind to several receptor tyrosine kinases, including receptors for epidermal growth factor, platelet-derived growth factor, and insulin as well as the long form of the leptin receptor, a glycoprotein 130-associated receptor. We now describe a novel member of this family, SNX6, which interacts with members of the transforming growth factor-␤ family of receptor serine-threonine kinases. These receptors belong to two classes: type II receptors that bind ligand, and type I receptors that are subsequently recruited to transduce the signal. Of the type II receptors, SNX6 was found to interact strongly with ActRIIB and more moderately with wild type and kinase-defective mutants of T␤RII. Of the type I receptors, SNX6 was found to interact only with inactivated T␤RI. SNXs 1-4 also interacted with the transforming growth factor-␤ receptor family, showing different receptor preferences. Conversely, SNX6 behaved similarly to the other SNX proteins in its interactions with receptor tyrosine kinases. Strong heteromeric interactions were also seen among SNX1, -2, -4, and -6, suggesting the formation in vivo of oligomeric complexes. These findings are the first evidence for the association of the SNX family of molecules with receptor serine-threonine kinases.The transforming growth factor-␤ (TGF-␤) 1 family includes a large number of peptides, including the TGF-␤s themselves, activin/inhibin, the bone morphogenetic proteins (BMPs), the growth and differentiation factors (GDFs), glial-derived neurotrophic factor, and Mü llerian inhibitory substance (1). Although there are no yeast TGF-␤s, homologs have been identified in primitive metazoans, including Caenorhabditis elegans and Drosophila (2-4). With the exception of only glial-derived neurotrophic factor, these ligands signal through heterotetrameric pairs of serine-threonine kinase receptors. Ligand first interacts with a type II receptor, which, following ligand binding, recruits a type I receptor (5). The type II receptors are constitutively active kinases, catalyzing phosphorylation both of themselves in an autocatalytic reaction and of the recruited type I receptor (6). Once bound to ligand and phosphorylated by the type II receptor, the type I receptor then transduces the signal to the intracellular signaling intermediates, including the recently described family of Smad proteins (7-13). In general, one or two closely related type I and one or two closely related type II receptors are utilized by each class of ligand. For example, TGF-␤1 and TGF-␤3 bind to the type II TGF-␤ receptor (T␤RII), with subsequent recruitment of the type I TGF-␤ receptor (T␤RI/ALK5 (activin-like kinase 5)) (6). Similarly, activin binds to either ActRII or ActRIIB, with activin type IB receptor (ALK4) or possi...

Regulation of cell proliferation by Smad proteins

“…Overexpression of Cav-1 was reported to reduce TGF-binduced Smad activation (Razani et al, 2001). Furthermore, the family of sorting nexins (SNXs) that are intracellular traf®cking molecules of receptor tyrosine kinases (Haft et al, 1998), also interact with TGF-b receptors (Parks et al, 2001).…”

Phosphoinositides in Constitutive Membrane Traffic

“…Subsequently, a number of SNX proteins have been shown to associate with specific membrane receptor proteins as part of an oligomeric complex that regulates sorting of receptors between recycling and degradation pathways (128,153,342). SNX proteins appear to associate into complexes with other SNX proteins, as well as with adaptor proteins that bind to receptors and to clathrin (193,206,211,267). The fraction of the genome devoted to SNX proteins in S. cerevisiae is threefold greater than it is in the human genome.…”

All Phox Homology (PX) Domains from Saccharomyces cerevisiae Specifically Recognize Phosphatidylinositol 3-Phosphate

“…The exception to this rule is SNX3 (26,27), which contains little more than just the PX domain and (in both yeast and mammals) lacks the coiled-coil regions proposed to drive sorting nexin oligomerization. Yhr105wp is the only other yeast-sorting nexin that lacks substantial sequence outside the PX domain and is also the only example other than SNX3 that has a high affinity PX domain.…”

“…oligomers (25)(26)(27)(28). The exception to this rule is SNX3 (26,27), which contains little more than just the PX domain and (in both yeast and mammals) lacks the coiled-coil regions proposed to drive sorting nexin oligomerization.…”