Sergei Yu. Venyaminov

Crystal structure of the α1β1 integrin I-domain: insights into integrin I-domain function

The α1β1 integrin is a major cell surface receptor for collagen. Ligand binding is mediated, in part, through a ∼200 amino acid inserted ‘I’‐domain contained in the extracellular part of the integrin α chain. Integrin I‐domains contain a divalent cation binding (MIDAS) site and require cations to interact with integrin ligands. We have determined the crystal structure of recombinant I‐domain from the rat α1β1 integrin at 2.2 Å resolution in the absence of divalent cations. The α1 I‐domain adopts the dinucleotide binding fold that is characteristic of all I‐domain structures that have been solved to date and has a structure very similar to that of the closely related α2β1 I‐domain which also mediates collagen binding. A unique feature of the α1 I‐domain crystal structure is that the MIDAS site is occupied by an arginine side chain from another I‐domain molecule in the crystal, in place of a metal ion. This interaction supports a proposed model for ligand‐induced displacement of metal ions. Circular dichroism spectra determined in the presence of Ca2+, Mg2+ and Mn2+ indicate that no changes in the structure of the I‐domain occur upon metal ion binding in solution. Metal ion binding induces small changes in UV absorption spectra, indicating a change in the polarity of the MIDAS site environment.

STRUCTURAL ANALYSIS OF THE A AND B CONFORMERS OF ESCHERICHIA COLI 5 S RIBOSOMAL RNA BY INFRARED SPECTROSCOPY

1. Introduction Rapid progress is being made in elucidating the structure of pro- and eukaryotic 5 S RNAs using dif- ferent physical, biochemical and sequence analysis approaches (reviewed in [ 11). It seems to be evident from comparative sequence studies [l-5] that a general base pairing scheme of the type first proposed in [2] is valid for eukaryotic 5 S RNAs and that a general secondary structure of the type first proposed in [3] extended by few base pairs is the structural basis for prokaryotic 5 S RNAs. Experimental evi- dence supporting these basic secondary structures is now manifold and derives, e.g., from such powerful specific techniques as high-resolution ‘H NMR spec- troscopy [6,7] and slow tritium exchange studies [8]. Summarizing data from optical ([6,9] and references within), infrared [lo], Raman [l l-131, and ‘H NMR spectroscopy [6,7] it became obvious that both for prokaryotic and eukaryotic 5 S RNA molecules in solution a highly ordered secondary/tertiary structure exists with an amount of about 35-42 base pairs (58-70% of all nucleotides are base-paired) in the presence of stabilizing ions. and the infrared thermal melting curves reveal partic- ular differences between both conformers at 570°C which may be useful with respect to the analysis of the intricate problem of the A-to-B conformational transition(s). 2. Materials and methods Here, we report the results of infrared spectro- scopic studies of the A and B conformers of

Identification and isolation of vinculin from platelets

A vinculin‐like protein was identified in chicken as well as in bovine platelets by ELISA competitive binding assay using antibodies against vinculin from chicken gizzard. By a modified procedure (J. Biol. Chem. (1980) 255, 1194–1199) we succeeded in isolating bovine platelet vinculin to apparent homogeneity. The structural identity of platelet and chicken gizzard vinculin was demonstrated by circular dichroism analysis. It was also shown that platelet vinculin induces a significant decrease in the low shear viscosity of F‐actin. Vinculin, in all probability, plays an important role in the organization of actin filaments in platelets, especially in the linkages of microfilaments to the membrane.