SignificanceMutants of RAS are major oncogenes and occur in many human cancers, but efforts to develop drugs that directly inhibit the corresponding constitutively active RAS proteins have failed so far. We therefore focused on SOS1, the guanine nucleotide exchange factor (GEF) and activator of RAS. A combination of high-throughput and fragment screening resulted in the identification of nanomolar SOS1 inhibitors, which effectively down-regulate active RAS in tumor cells. In cells with wild-type KRAS, we observed complete inhibition of the RAS-RAF-MEK-ERK pathway. In a mutant KRAS cell line, SOS1 inhibition resulted in a reduction of phospho-ERK activity by 50%. Together, the data indicate that inhibition of GEFs may represent a viable approach for targeting RAS-driven tumors.

Adhesive interactions involving CD44, the cell surface receptor for hyaluronan, underlie fundamental processes such as inflammatory leukocyte homing and tumor metastasis. Regulation of such events is critical and appears to be effected by changes in CD44 N-glycosylation that switch the receptor "on" or "off" under appropriate circumstances. How altered glycosylation influences binding of hyaluronan to the lectin-like Link module in CD44 is unclear, although evidence suggests additional flanking sequences peculiar to CD44 may be involved. Here we show using X-ray crystallography and NMR spectroscopy that these sequences form a lobular extension to the Link module, creating an enlarged HA binding domain and a formerly unidentified protein fold. Moreover, the disposition of key N-glycosylation sites reveals how specific sugar chains could alter both the affinity and avidity of CD44 HA binding. Our results provide the necessary structural framework for understanding the diverse functions of CD44 and developing novel therapeutic strategies.

The solution structure of the Link module from human TSG-6, a hyaladherin with important roles in inflammation and ovulation, has been determined in both its free and hyaluronan-bound conformations. This reveals a well defined hyaluronan-binding groove on one face of the Link module that is closed in the absence of ligand. The groove is lined with amino acids that have been implicated in mediating the interaction with hyaluronan, including two tyrosine residues that appear to form essential intermolecular hydrogen bonds and two basic residues capable of supporting ionic interactions. This is the first structure of a non-enzymic hyaladherin in its active state, and identifies a ligand-induced conformational change that is likely to be conserved across the Link module superfamily. NMR and isothermal titration calorimetry experiments with defined oligosaccharides have allowed us to infer the minimum length of hyaluronan that can be accommodated within the binding site and its polarity in the groove; these data have been used to generate a model of the complex formed between the Link module and a hyaluronan octasaccharide.Hyaluronan (HA), 1 a high molecular weight polysaccharide with a central role in extracellular matrix organization and cell adhesion in mammals (1), is essential to a wide range of normal physiological processes including development, immunology, and reproduction (2-4). Alterations in the metabolism and localization of this molecule underlie the progression of many diseases, for instance arthritis, pulmonary/vascular disorders, and cancer (5, 6). These diverse biological activities may seem surprising for a linear polymer composed entirely of a repeating disaccharide (i.e. -glucuronic acid-␤Ϫ1,3-N-acetylglucosamine-␤Ϫ1,4-; up to 10 7 Da) that, unlike other glycosaminoglycans, is neither attached to a core protein nor sulfated. This functional complexity is thought to arise from the interaction of HA with a large number of specific HA-binding proteins (7), which can form structurally diverse complexes (see Ref. 8). The majority of these "hyaladherins" belong to a superfamily of proteins that share a common ϳ100 amino acid domain, termed a Link module, that mediates the interaction with HA.Previously we have determined the solution structure of the Link module from human TSG-6 (the protein product of the tumor necrosis factor-stimulated gene-6 (9)), thereby defining the consensus fold for this superfamily (10). In TSG-6, a 35-kDa secreted protein composed mainly of contiguous Link and CUB modules, the Link module is sufficient to mediate a high affinity interaction with HA (10, 11); this has been termed a "type A" HA-binding domain (7). The HA receptor CD44, which has an important role in mediating lymphocyte migration, however, requires N-and C-terminal extensions to its Link module for correct folding and functional activity of its type B interaction domain. Most other members of the superfamily, such as link proteins and chondroitin-sulfate proteoglycans (critical for extracellular matrix organiza...

The Fyn SH3 domain has a well-defined structure in solution. The relative binding affinities of the three ligand peptides and their orientation within the Fyn SH3 complex were consistent with recently proposed models for the binding of 'consensus' polyproline sequences. Although the affinities of consensus and non-consensus peptides are different, the degree of difference is not very large, suggesting that SH3 domains bind to polyproline peptides in a promiscuous manner.