Our comparison reveals that the overall structure of OmpF is not influenced by crystal lattice constraints and, thus, presumably bears close resemblance to the in vivo structure. The tetragonal crystal structure has provided the starting model for the phasing of neutron diffraction data obtained from this crystal form, as described in an accompanying article.
The medium chain mu 2 subunit (AP50) of the clathrin-associated adapter protein complex 2 (AP-2) interacts specifically with the tyrosine-based signals of several integral membrane proteins through the consensus sequence YXXPhi, where X can be any residue and Phi is a large hydrophobic residue. Using surface plasmon resonance combined with structural information, we have analysed the interaction of AP50 with peptides derived from the cytoplasmic tail of cytotoxic T-lymphocyte antigen 4 (CTLA-4). The crystal structure of AP50 in complex with a CTLA-4-derived peptide was determined to 3.6 A (1 A=0.1 nm) resolution. The binding domain of AP50 (residues 164-435) was expressed in Escherichia coli and purified. In agreement with previous reports, the AP50 domain bound to residues 152-174 of CTLA-4, but not to the same peptide that was phosphorylated at the single tyrosine residue (position 165). The interaction exhibited fast kinetics with rapid on and off rates and a K(d) of 0.7 microM. In order to further understand why AP50 binds to CTLA-4, but not to the homologous receptor CD28, a comparison of binding of AP50 with five peptides with single changes in and around the YXXPhi motif to the equivalent residues of CD28 was made. T162H greatly reduced binding, whereas T161L had little effect. Mutations G163S, V164D and K167N all exhibited reduced binding. Modelling of the single amino acid changes using structural information, was in broad agreement with the binding data, demonstrating that residues outside of the YXXPhi motif are also important in the interaction of membrane proteins with AP50.
The structure of l9αH-lupeol
methyl ether isolated from Chionochloa bromoides has been determined by 13C n.m.r.
spectroscopy and X-ray diffraction. Crystals of the triterpene
are monoclinic, space group P21, with a 8.071(1), b
14.5477(6), c 11.6061(4) �, β
104.466(5)�, V 319.6 �3 and Z 2. The structure was solved by direct
methods, and was refined to R 0.054 for 1092 observed reflections. Previous
reports of synthetic and naturally occurring 19αH-lupanes are briefly
discussed, as is the distribution of the chemotaxonomically
discriminating triterpene methyl ethers in Chionochloa.
The X-ray crystal structure
of 3α,4α:5β,6β-diepoxyandrostan-17-one has been determined.
Crystals of the title compound (C19H26O3)are
monoclinic, space group P21, with a 9.208(2), b 9.620(4), c 9.312(3) �, β 99.14(2)�, V 814.5
Ǻ3 and Z 2. The structure was solved by direct methods and refined
to R 0.039 for 887 observed reflexions. The 3α,4α:5β,6β
configuration of the epoxide rings confirms the assignment based on proton
n.m.r. studies.
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