NMR and mutagenesis evidence for an I domain allosteric site that regulates lymphocyte function-associated antigen 1 ligand binding

The leukocyte integrin, lymphocyte function-associated antigen 1 (LFA-1) (CD11a/CD18), mediates cell adhesion and signaling in inflammatory and immune responses. To support these functions, LFA-1 must convert from a resting to an activated state that avidly binds its ligands such as intercellular adhesion molecule 1 (ICAM-1). Biochemical and x-ray studies of the Mac-1 (CD11b/CD18) I domain suggest that integrin activation could involve a conformational change of the C-terminal alpha-helix. We report the use of NMR spectroscopy to identify CD11a I domain residues whose resonances are affected by binding to ICAM-1. We observed two distinct sites in the CD11a I domain that were affected. As expected from previous mutagenesis studies, a cluster of residues localized around the metal ion-dependent adhesion site (MIDAS) was severely perturbed on ICAM-1 binding. A second cluster of residues distal to the MIDAS that included the C-terminal alpha-helix of the CD11a I domain was also affected. Substitution of residues in the core of this second I domain site resulted in constitutively active LFA-1 binding to ICAM-1. Binding data indicates that none of the 20 substitution mutants we tested at this second site form an essential ICAM-1 binding interface. We also demonstrate that residues in the I domain linker sequences can regulate LFA-1 binding. These results indicate that LFA-1 binding to ICAM-1 is regulated by an I domain allosteric site (IDAS) and that this site is structurally linked to the MIDAS.

NMR structure and mutagenesis of the N-terminal Dbl homology domain of the nucleotide exchange factor Trio

Guanine nucleotide exchange factors for the Rho family of GTPases contain a Dbl homology (DH) domain responsible for catalysis and a pleckstrin homology (PH) domain whose function is unknown. Here we describe the solution structure of the N-terminal DH domain of Trio that catalyzes nucleotide exchange for Rac1. The all-alpha-helical protein has a very different structure compared to other exchange factors. Based on site-directed mutagenesis, functionally important residues of the DH domain were identified. They are all highly conserved and reside in close proximity on two a helices. In addition, we have discovered a unique capability of the PH domain to enhance nucleotide exchange in DH domain-containing proteins.

Solution structure of the cytohesin-1 (B2-1) Sec7 domain and its interaction with the GTPase ADP ribosylation factor 1

Cytohesin-1 (B2-1) is a guanine nucleotide exchange factor for human ADP ribosylation factor (Arf) GTPases, which are important for vesicular protein trafficking and coatamer assembly in the cell. Cytohesin-1 also has been reported to promote cellular adhesion via binding to the beta2 integrin cytoplasmic domain. The solution structure of the Sec7 domain of cytohesin-1, which is responsible for both the protein's guanine nucleotide exchange factor function and beta2 integrin binding, was determined by NMR spectroscopy. The structure consists of 10 alpha-helices that form a unique tertiary fold. The binding between the Sec7 domain and a soluble, truncated version of human Arf-1 was investigated by examining 1H-15N and 1H-13C chemical shift changes between the native protein and the Sec7/Arf-1 complex. We show that the binding to Arf-1 occurs through a large surface on the C-terminal subdomain that is composed of both hydrophobic and polar residues. Structure-based mutational analysis of the cytohesin-1 Sec7 domain has been used to identify residues important for binding to Arf and for mediating nucleotide exchange. Investigations into the interaction between the Sec7 domain and the beta2 integrin cytoplasmic domain suggest that the two proteins do not interact in the solution phase.

Identification and characterisation of a human calmodulin-stimulated phosphodiesterase PDE1B1

A cDNA encoding a calmodulin-stimulated 3',5'-cyclic nucleotide phosphodiesterase (PDE) was isolated from a human brain cDNA library. The cDNA, designated HSPDE1B1, encoded a protein of 536 amino acids that shared 96% sequence identity with the bovine "63 kDa" calmodulin-stimulated PDE. The recombinant protein had cyclic nucleotide phosphodiesterase activity that was stimulated approximately 2-fold by Ca2+/calmodulin and preferred cGMP as substrate. In addition, the enzymatic activity of HSPDE1B1 was inhibited by phosphodiesterase inhibitors with potencies similar to that displayed toward the bovine PDE1 enzymes: IBMX approximately equal to 8-methoxymethyl-IBMX > vinpocetine approximately equal to zaprinast > cilostamide > rolipram. HSPDE1B1 mRNA was found predominantly in the brain. Lower mRNA levels were found in heart and skeletal muscle. In situ hybridisation of brain revealed expression of HSPDE1B1 predominately in neuronal cells of the cerebellum, hippocampus and caudate. The HSPDE1B1 gene was mapped to human chromosome 12. A partial genomic sequence of HSPDE1B1 was isolated and shown to contain two splice junctions that are conserved in the rat PDE4 and the Drosophila dunce genes.

The reliability and validity of work measurement in Australian general practice consultations

OBJECTIVES

To examine the reliability of relative work value assessment in general practice consultations and to determine whether different methods of assessing work produce consistent rankings.

DESIGN

Cross-sectional observational assessment of general practice consulations.

SETTING

General practices in Victoria between October 1991 and October 1992.

PARTICIPANTS

686 patients attending one of 58 general practitioners (GPs) drawn from a random, stratified sample.

METHODS

Each participating GP had one day of consultations videotaped. They rated the work value of each consultation by using a magnitude estimation scale relative to a reference vignette. Three GP observers independently applied the same scale to the videotaped consultations. After three months, the observers applied a second measurement of work value, a compensation scale (also relative to the reference vignette), to the videotaped consultations. Duration of consultation was the third rating method.

MAIN OUTCOME MEASURES

The reliability of work value assessment for each scale. Consultation rank order correlation coefficients among all rating methods.

RESULTS

Observer reliability was high for both scales. Practising GPs showed lower levels of reliability in assessing the work value of their consultations. Strong positive correlations were found for consultation rankings among the observer scales and duration of the consultation. The duration of the consultation emerged as an important predictor of consultation work value.

CONCLUSIONS

Scaling methods appear to be of little value to the practising GP in reliably assessing the relative work value of their consultations; training in the use of these scales may improve their reliability. However, the duration of consultation may be a reasonable proxy for relative work value assessment in general practice consultations.

Isolation and characterization of cDNAs corresponding to two human calcium, calmodulin-regulated, 3',5'-cyclic nucleotide phosphodiesterases

cDNAs corresponding to two human calcium, calmodulin (CaM)-regulated 3',5'-cyclic nucleotide phosphodiesterases (PDEs) were isolated. One, Hcam1 (PDE1A3), corresponds to the bovine 61-kDa CaM PDE (PDE1A2). The second, Hcam3 (PDE1C), represents a novel phosphodiesterase gene. Hcam1 encodes a 535-amino acid protein that differs most notably from the bovine 61-kDa CaM PDE by the presence of a 14-amino acid insertion and a divergent carboxyl terminus. RNase protection studies indicated that Hcam1 is represented in human RNA from several tissues, including brain, kidney, testes, and heart. Two carboxyl-terminal splice variants for Hcam3 were isolated. One, Hcam3b (PDE1C1), encodes a protein 634 amino acids (72 kDa) in length. The other, Hcam3a (PDE1C3), diverges from Hcam3b 4 amino acids from the carboxyl terminus of Hcam3b, and extends an additional 79 amino acids. All the cDNAs isolated for Hcam3a are incomplete; they do not include the 5'-end of the open reading frame. Northern analysis revealed that both splice variants were expressed in several tissues, including brain and heart, and that there may be additional splice variants. Amino-truncated recombinant proteins were expressed in yeast and characterized biochemically. Hcam3a has a high affinity for both cAMP and cGMP and thus has distinctly different kinetic parameters from Hcam1, which has a higher affinity for cGMP than for cAMP. Both PDE1C enzymes were inhibited by isobutylmethylxanthine, 8-methoxymethyl isobutylmethylxanthine, zaprinast, and vinpocetine.

Mass mortality of common dolphins (Delphinus delphis) in south west England due to incidental capture in fishing gear

In the first quarter of 1992, 118 dolphin carcases, of which 54 were positively identified as common dolphins (Delphinus delphis), were found stranded on the coast of Cornwall and Devon. To determine the cause, detailed post mortem examinations were carried out on 38 of the carcases, and the results were compared with those from 20 common dolphins that stranded on the coast of Cornwall and Devon in the previous 15 months. There was no evidence that the deaths were due to an infectious or parasitic disease, or acute intoxication by any of the algal toxins, trace metals or chlorinated hydrocarbons measured. However, 30 of the 38 dolphins showed signs associated with incidental capture in fishing gear. Skin lesions characteristic of capture in a small-meshed net and the predominance of recently ingested Atlantic mackerel (Scomber scombrus) and pilchard (Sardina pilchardus) in the stomachs of the dolphins suggested that they had been caught in the trawl or purse seine nets used for these fish. There is insufficient information to explain why this high mortality occurred in 1992 and not in other years.