The polypyrimidine tract binding protein (PTB) is an important regulator of alternative splicing that also affects mRNA localization, stabilization, polyadenylation, and translation. NMR structural analysis of the N-terminal half of PTB (residues 55-301) shows a canonical structure for RRM1 but reveals novel extensions to the beta strands and C terminus of RRM2 that significantly modify the beta sheet RNA binding surface. Although PTB contains four RNA recognition motifs (RRMs), it is widely held that only RRMs 3 and 4 are involved in RNA binding and that RRM2 mediates homodimerization. However, we show here not only that the RRMs 1 and 2 contribute substantially to RNA binding but also that full-length PTB is monomeric, with an elongated structure determined by X-ray solution scattering that is consistent with a linear arrangement of the constituent RRMs. These new insights into the structure and RNA binding properties of PTB suggest revised models of its mechanism of action.

The L1 protuberance of the 50S ribosomal subunit is implicated in the release/disposal of deacylated tRNA from the E site. The apparent mobility of this ribosomal region has thus far prevented an accurate determination of its three-dimensional structure within either the 50S subunit or the 70S ribosome. Here we report the crystal structure at 2.65 A resolution of ribosomal protein L1 from Sulfolobus acidocaldarius in complex with a specific 55-nucleotide fragment of 23S rRNA from Thermus thermophilus. This structure fills a major gap in current models of the 50S ribosomal subunit. The conformations of L1 and of the rRNA fragment differ dramatically from those within the crystallographic model of the T. thermophilus 70S ribosome. Incorporation of the L1-rRNA complex into the structural models of the T. thermophilus 70S ribosome and the Deinococcus radiodurans 50S subunit gives a reliable representation of most of the L1 protuberance within the ribosome.

Vascular endothelial growth factor (VEGF)-D is a secreted glycoprotein that induces angiogenesis and lymphangiogenesis. It consists of a central domain, containing binding sites for VEGF receptor-2 (VEGFR-2) and VEGFR-3, and N- and C-terminal propeptides. It is secreted from the cell as homodimers of the full-length form that can be proteolytically processed to remove the propeptides. It was recently shown, using adenoviral gene delivery, that fully processed VEGF-D induces angiogenesis in vivo, whereas full-length VEGF-D does not. To better understand these observations, we monitored the effect of VEGF-D processing on receptor binding using a full-length VEGF-D mutant that cannot be processed. This mutant binds VEGFR-2, the receptor signaling for angiogenesis, with approximately 17,000-fold lower affinity than mature VEGF-D, indicating the importance of processing for interaction with this receptor. Further, we show that members of the proprotein convertase (PC) family of proteases promote VEGF-D processing, which facilitates the VEGF-D/VEGFR-2 interaction. The PCs furin and PC5 promote cleavage of both propeptides, whereas PC7 promotes cleavage of the C-terminal propeptide only. The finding that PCs promote activation of VEGF-D and other proteins with roles in cancer such as matrix metalloproteinases, emphasizes the importance of these enzymes as potential regulators of tumor progression and metastasis.

. The extensive surface area of L22 has the characteristics of an RNA-binding protein, consistent with its role in the folding of the 23S rRNA. The erythromycin-resistance conferring mutation is located in the protruding beta hairpin that is postulated to be important in L22-rRNA interactions. This region of the protein might be at the erythromycin-binding site close to the peptidyl transferase center, whereas the opposite end may be exposed to the cytoplasm.

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