Type I interferons (IFNs) 1 consisting of α and β subtypes, are a multifunctional cytokine family capable of inhibiting cell proliferation and viral replication, in addition to modulating cellular immune functions. The type I IFN receptor consists of at least two distinct subunits IFNAR1 and IFNAR2. IFNAR1 is the low-affinity subunit, originally cloned in 1990, and is composed of four fibronectin type II-like (FNII-like) subdomains, termed SD1-4. Type I IFNs bind SD1-3 with a typically high binding affinity which is between 0.5–5 μM; IFNα1 and IFNβ are exceptions with higher binding affinities, up to 220 nM and 100 nM, respectively. Type I IFNs have a binding association rate of 5x105M/s with a variable dissociation rate that determines type I IFN subtype affinity for IFNAR1. IFNAR2 was originally cloned in 1994. It is the high-affinity subunit, composed of two FNII-like subdomains, termed D1 and D2. Type I IFNs bind D1 and D2 with a typical binding affinity between 0.4-5 nM; IFNβ binds at a slightly lower affinity (0.1 nM).
The IFNAR1 chain appears to be involved primarily in signal transduction, while the IFNAR2 chain plays a role both in ligand binding and signal transduction. The molecular weight of the IFNAR2 chain was approximately 50 kDa (IFNAR2.1), having a truncated cytoplasmic domain due to alternative mRNA splicing. In most human cells IFNAR2.1 is expressed at low levels relative to the full-length IFNAR2.2 chain.
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