Mutational Analysis of the Fractalkine Chemokine Domain: BASIC AMINO ACID RESIDUES DIFFERENTIALLY CONTRIBUTE TO CX3CR1 BINDING, SIGNALING, AND CELL ADHESION

Jeffrey K Harrison,Alanm Fong,Petera.W Swain,Shuzhen Chen,Yen-Reia Yu,Mina N Salafranca,William B Greenleaf,Toshio Imai,Dhavalkumar D Patel

doi:10.1074/jbc.m010261200

Abstract

Fractalkine (FKN/CX3CL1) is a unique member of the chemokine gene family and contains a chemokine domain (CD), a mucin-like stalk, a single transmembrane region, and a short intracellular C terminus. This structural distinction affords FKN the property of mediating capture and firm adhesion of FKN receptor (CX3CR1)-expressing cells under physiological flow conditions. Shed forms of FKN also exist, and these promote chemotaxis of CX3CR1-expressing leukocytes. The goal of the present study was to identify specific residues within the FKN-CD critical for FKN-CX3CR1 interactions. Two residues were identified in the FKN-CD, namely Lys-7 and Arg-47, that are important determinants in mediating an FKN-CX3CR1 interaction. FKN-K7A and FKN-R47A mutants exhibited 30-60-fold decreases in affinity for CX3CR1 and failed to arrest efficiently CX3CR1-expressing cells under physiological flow conditions. However, these mutants had differential effects on chemotaxis of CX3CR1-expressing cells. The FKN-K7A mutant acted as an equipotent partial agonist, whereas the FKN-R47A mutant had marked decreased potency and efficacy in measures of chemotactic activity. These data identify specific structural features of the FKN-CD that are important in interactions with CX3CR1 including steady state binding, signaling, and firm adhesion of CX3CR1-expressing cells.

Highlights

Fractalkine (FKN/CX3CL1) is a unique member of the chemokine gene family and contains a chemokine domain (CD), a mucin-like stalk, a single transmembrane region, and a short intracellular C terminus
Our first goal was directed toward determining the effect of various lengths of this mucin-like domain on the affinity and efficacy of the FKN-CD for binding and activating CX3CR1
In order to evaluate the effect of various lengths of the mucin stalk on FKN-CD function, five truncation mutants of human FKN were prepared in which DNA sequences encoding the signal peptide, the chemokine domain, and differential lengths of the mucin stalk were cloned in frame with C-terminal six histidine residues (His6)

Summary

Introduction

Fractalkine (FKN/CX3CL1) is a unique member of the chemokine gene family and contains a chemokine domain (CD), a mucin-like stalk, a single transmembrane region, and a short intracellular C terminus This structural distinction affords FKN the property of mediating capture and firm adhesion of FKN receptor (CX3CR1)expressing cells under physiological flow conditions. The FKN-K7A mutant acted as an equipotent partial agonist, whereas the FKN-R47A mutant had marked decreased potency and efficacy in measures of chemotactic activity These data identify specific structural features of the FKN-CD that are important in interactions with CX3CR1 including steady state binding, signaling, and firm adhesion of CX3CR1-expressing cells. We employed deletional and site-directed mutagenesis in order to evaluate the effect of different lengths of the mucin stalk and determine the role of specific amino acid residues present in the FKN chemokine domain, in mediating FKN binding, signaling, and CX3CR1-dependent cell adhesion

Objectives

Results

Conclusion