A variant of the PTPN22-encoded Lyp phosphatase (Lyp620W) confers risk for autoimmune disease, but the mechanisms underlying this association remain unclear. We show here that mice expressing the Lyp variant homolog Pep619W manifest thymic and splenic enlargement accompanied by increases in T-cell number, activation and positive selection and in dendritic- and B-cell activation. Although Ptpn22 (Pep) transcript levels were comparable in Pep619W and wild-type Pep619R mice, Pep protein levels were dramatically reduced in the mutant mice, with Pep619W protein being more rapidly degraded and showing greater association with and in vitro cleavage by calpain 1 than Pep619R. Similarly, levels of the Lyp620W variant were decreased in human T and B cells, and its calpain binding and cleavage were increased relative to wild-type Lyp620R. Thus, calpain-mediated degradation with consequently reduced Lyp/Pep expression and lymphocyte and dendritic cell hyperresponsiveness represents a mechanism whereby Lyp620W may increase risk for autoimmune disease.

The Hedgehog (Hh) signaling pathway has critical functions during embryogenesis of both invertebrate and vertebrate species [1]; defects in this pathway in humans can cause developmental disorders as well as neoplasia [2]. Although the Gli1, Gli2, and Gli3 zinc finger proteins are known to be effectors of Hh signaling in vertebrates, the mechanisms regulating activity of these transcription factors remain poorly understood [3] [4]. In Drosophila, activity of the Gli homolog Cubitus interruptus (Ci) is likely to be modulated by its interaction with a cytoplasmic complex containing several other proteins [5] [6], including Costal2, Fused (Fu), and Suppressor of fused (Su(fu)), the last of which has been shown to interact directly with Ci [7]. We have cloned mouse Suppressor of fused (mSu(fu)) and detected its 4.5 kb transcript throughout embryogenesis and in several adult tissues. In cultured cells, mSu(fu) overexpression inhibited transcriptional activation mediated by Sonic hedgehog (Shh), Gli1 and Gli2. Co-immunoprecipitation of epitope-tagged proteins indicated that mSu(fu) interacts with Gli1, Gli2, and Gli3, and that the inhibitory effects of mSu(fu) on Gli1's transcriptional activity were mediated through interactions with both amino- and carboxy-terminal regions of Gli1. Gli1 was localized primarily to the nucleus of both HeLa cells and the Shh-responsive cell line MNS-70; co-expression with mSu(fu) resulted in a striking increase in cytoplasmic Gli1 immunostaining. Our findings indicate that mSu(fu) can function as a negative regulator of Shh signaling and suggest that this effect is mediated by interaction with Gli transcription factors.

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...