Structure of Natural Killer Receptor 2B4 Bound to CD48 Reveals Basis for Heterophilic Recognition in Signaling Lymphocyte Activation Molecule Family

Abstract

Natural killer (NK) cells eliminate virally infected and tumor cells. Among the receptors regulating NK cell function is 2B4 (CD244), a member of the signaling lymphocyte-activation molecule (SLAM) family that binds CD48. 2B4 is the only heterophilic receptor of the SLAM family, whose other members, e.g., NK-T-B-antigen (NTB-A), are self-ligands. We determined the structure of the complex between the N-terminal domains of mouse 2B4 and CD48, as well as the structures of unbound 2B4 and CD48. The complex displayed an association mode related to, yet distinct from, that of the NTB-A dimer. Binding was accompanied by the rigidification of flexible 2B4 regions containing most of the polymorphic residues across different species and receptor isoforms. We propose a model for 2B4-CD48 interactions that permits the intermixing of SLAM receptors with major histocompatibility complex-specific receptors in the NK cell immune synapse. This analysis revealed the basis for heterophilic recognition within the SLAM family.

Functional role of human NK cell receptor 2B4 (CD244) isoforms

“…At one corner of the interface, the FG loop of 2B4 makes six hydrogen bonds with the CC 0 loop of CD48, all involving main chain atoms of 2B4. At the opposite corner, the FG loop of CD48 engages the CC 0 loop of 2B4 through a combination of two salt bridges and five hydrogen bonds, as well as numerous main chain-main chain contacts [26]. In our study, the presence of extra five amino acids viz Glu127, Ser128, Leu129, Leu130 and Pro131 in h2B4-B causes an additional turn in the strands at the junction of V and C2 domain.…”

“…3C and D). The 3-D structure of 2B4-CD48 complex suggests that the FG-loop of 2B4 is located very close to the CC 0 -loop of CD48 and is involved in the 2B4-CD48 interaction [26,31]. Therefore, the change in the folding and conformation of h2B4-B could potentially alter the receptor-ligand binding resulting in its reduced binding to CD48 observed in our study.…”

“…In the 2B4-CD48 complex, the AGFCC 0 C 00 b sheet of 2B4 interacts face to face with the corresponding b sheet of CD48, such that the two sheets contact each other at an $751 angle [26]. This association mode shows overall similarities to that observed in the CD2-CD58 heterophilic complex [33], but it differs in the detailed architecture and physicochemical properties of the interface.…”

“…Molecular characterization of the genomic structure revealed that both isoforms are encoded by the same gene. The differences in the human isoforms arise by differential splicing that result in an addition of five amino acids at the immunoglobulin V domain in the extracellular region [29].In the 2B4-CD48 complex, the AGFCC 0 C 00 b sheet of 2B4 interacts face to face with the corresponding b sheet of CD48, such that the two sheets contact each other at an $751 angle [26]. This association mode shows overall similarities to that observed in the CD2-CD58 heterophilic complex [33], but it differs in the detailed architecture and physicochemical properties of the interface.…”

“…Although, recent reports suggest that 2B4 in humans and mice may have dual function of a single receptor-ligand pair that is regulated by cell surface density, degree of cross-linking, and the abundance or limitation of adaptor molecules [25]. It seems likely that human and mouse 2B4 engages CD48 with similar docking topologies [26]. Moreover, the human and mouse 2B4-CD48 interactions have comparable affinities [16].…”