Adaptation to hypoxia is mediated by transactivation of hypoxiaresponsive genes by hypoxia-inducible factor-1 (HIF-1) in complex with the CBP and p300 transcriptional coactivators. We report the solution structure of the cysteine͞histidine-rich 1 (CH1) domain of p300 bound to the C-terminal transactivation domain of HIF-1␣. CH1 has a triangular geometry composed of four ␣-helices with three intervening Zn 2؉ -coordinating centers. CH1 serves as a scaffold for folding of the HIF-1␣ C-terminal transactivation domain, which forms a vise-like clamp on the CH1 domain that is stabilized by extensive hydrophobic and polar interactions. The structure reveals the mechanism of specific recognition of p300 by HIF-1␣, and shows how HIF-1␣ transactivation is regulated by asparagine hydroxylation.

Formin proteins participate in a wide range of cytoskeletal processes in all eukaryotes. The defining feature of formins is a highly conserved approximately 400 residue region, the Formin Homology-2 (FH2) domain, which has recently been found to nucleate actin filaments. Here we report crystal structures of the S. cerevesiae Bni1p FH2 domain. The mostly alpha-helical FH2 domain forms a unique "tethered dimer" in which two elongated actin binding heads are tied together at either end by an unusual lasso and linker structure. Biochemical and crystallographic observations indicate that the dimer is stable but flexible, with flexibility between the two halves of the dimer conferred by the linker segments. Although each half of the dimer is competent to interact with filament ends, the intact dimer is required for actin nucleation and processive capping. The tethered dimer architecture may allow formins to stair-step on the barbed end of an elongating nascent filament.

SAP (SLAM-associated protein) is a small lymphocyte-specific signalling molecule that is defective or absent in patients with X-linked lymphoproliferative syndrome (XLP). Consistent with its single src homology 2 (SH2) domain architecture and unusually high affinity for SLAM (also called CD150), SAP has been suggested to function by blocking binding of SHP-2 or other SH2-containing signalling proteins to SLAM receptors. Additionally, SAP has recently been shown to be required for recruitment and activation of the Src-family kinase FynT after SLAM ligation. This signalling 'adaptor' function has been difficult to conceptualize, because unlike typical SH2-adaptor proteins, SAP contains only a single SH2 domain and lacks other recognized protein interaction domains or motifs. Here, we show that the SAP SH2 domain binds to the SH3 domain of FynT and directly couples FynT to SLAM. The crystal structure of a ternary SLAM-SAP-Fyn-SH3 complex reveals that SAP binds the FynT SH3 domain through a surface-surface interaction that does not involve canonical SH3 or SH2 binding interactions. The observed mode of binding to the Fyn-SH3 domain is expected to preclude the auto-inhibited conformation of Fyn, thereby promoting activation of the kinase after recruitment. These findings broaden our understanding of the functional repertoire of SH3 and SH2 domains.