Analysis of the C5a anaphylatoxin core domain using a C5a phage library selected on differentiated U937 cells

Allosterism in human complement component 5a ((h)C5a): a damper of C5a receptor (C5aR) signaling

The phenomena of allosterism continues to advance the field of drug discovery, by illuminating gainful insights for many key processes, related to the structure-function relationships in proteins and enzymes, including the transmembrane G-protein coupled receptors (GPCRs), both in normal as well as in the disease states. However, allosterism is completely unexplored in the native protein ligands, especially when a small covalent change significantly modulates the pharmacology of the protein ligands toward the signaling axes of the GPCRs. One such example is the human C5a ((h)C5a), the potent cationic anaphylatoxin that engages C5aR and C5L2 to elicit numerous immunological and non-immunological responses in humans. From the recently available structure-function data, it is clear that unlike the mouse C5a ((m)C5a), the (h)C5a displays conformational heterogeneity. However, the molecular basis of such conformational heterogeneity, otherwise allosterism in (h)C5a and its precise contribution toward the overall C5aR signaling is not known. This study attempts to decipher the functional role of allosterism in (h)C5a, by exploring the inherent conformational dynamics in (m)C5a, (h)C5a and in its point mutants, including the proteolytic mutant des-Arg(74)-(h)C5a. Prima facie, the comparative molecular dynamics study, over total 500 ns, identifies Arg(74)-Tyr(23) and Arg(37)-Phe(51) "cation-π" pairs as the molecular "allosteric switches" on (h)C5a that potentially functions as a damper of C5aR signaling.

Characterisation of C5a receptor agonists from phage display libraries

C5a des-Arg(74) has a 10- to 100-fold lower receptor binding affinity than intact C5a and is only a partial agonist. We have used phage display selection from randomly mutated C5a des-Arg(74) libraries to isolate variant proteins that can activate C5a receptors with similar potency to C5a. Here we explore the interactions of three variants (V1-3) with C5aR mutated at residues involved in the differential response. The mutant Asp(282)Arg-C5aR is preferentially activated by C5a des-Arg(74), probably due to repulsion between Arg(74) of C5a and the substituent Arg(282). In accordance with this hypothesis, V2 (with a polar C-terminus which has no Arg residue) but not V1 (with a C-terminal Arg residue at position 73) could activate Asp(282)Arg-C5aR. V3, with a very hydrophobic C-terminus, was the most potent agonist at Asp(282)Arg-C5aR. Arg(175) is a potential counterion for the C-terminal carboxylate of C5a. C5aR mutated to either Ala or Asp at this position lost nearly all responsiveness to both C5a and C5a des-Arg(74), suggesting that mutation of Arg(175) caused a non-specific loss of receptor conformation and a loss of signalling capacity. However, V3 could still activate Arg(175)Asp/Ala-C5aR with the same potency as wild-type C5aR, demonstrating that the mutant receptors retained high signalling capability and showed a specific loss of responsiveness. Thus C5a des-Arg(74) variants produced by phage display are potentially useful tools for the dissection of ligand-receptor interactions.

The complement component C5 of the common carp (Cyprinus carpio): cDNA cloning of two distinct isotypes that differ in a functional site

The complement component C5 plays important roles in inflammatory responses and complement-mediated cytolysis. In bony fish, although C5 has been identified at the DNA or the protein level in trout, carp and gilthead seabream, only partial C5 sequences are available. The present study was designed to obtain the complete primary structure of C5 from the common carp ( Cyprinus carpio) and to examine its possible structural diversity. Reverse-transcribed polymerase chain reaction amplification from carp hepatopancreatic RNA resulted in isolation of six distinct C5-like cDNA segments, which were grouped into two divergent types (type I and type II). Using two sequences representative of the two types as probes, two distinct full-length cDNA clones (C5-1 and C5-2) were isolated, in addition to a truncated isoform of C5-1 (C5-1'). The deduced amino acid sequences of C5-1 and C5-2 share 83% identity and predict a typical two-chain structure of the mature protein that lacks the thioester bond, as in C5 from other animals. Southern hybridization of genomic DNA suggested the presence of multiple genes encoding C5-type I and a single gene encoding C5-type II. Interestingly, carp C5-type I contains novel subtypes like C5-1 that have a histidine instead of the well-conserved arginine at the cleavage site for the C5 convertase, both in the complete and truncated forms. Northern blotting analysis suggested that C5-type I and C5-type II are mainly expressed in hepatopancreas, and the expression levels are significantly increased by stimulating carp with lipopolysaccharide or beta-1,3-glucan. Possible functional divergence among the C5 isotypes in carp is discussed.

Selection of rotavirus VP4 cell receptor binding domains for MA104 cells using a phage display library

Rotavirus infection of host cells, like other viruses, is a complex process that has not been fully elucidated, and much attention has been focused on the regions of the viral attachment protein, VP4, that are involved in binding to the cellular receptor. In this study, phage display technology was employed to generate a g3p VP4 gene-targeted phage display peptide library using the porcine rotavirus strain CRW8, and a method was optimised for panning this library on adherent MA104 cells to identify receptor binding domains. Recombinant phage that displayed expressed peptides from both the rotavirus VP4 trypsin cleavage products VP8* and VP5* were selected, and while some of the phage clones contained insert sequences from regions of VP4 implicated previously in cell binding and infection, new domains were also identified. In all, four regions within VP8* and six regions of VP5* were selected by panning. To our knowledge, this paper is the first description of using a gene-targeted phage display library to identify receptor binding domains on viral proteins.