Comparative analysis of structure, expression and PSD95-binding capacity of Lrfn, a novel family of neuronal transmembrane proteins

Leucine-rich repeat and fibronectin III domain-containing (Lrfn) has five members in mouse and human (Lrfn1, Lrfn2, Lrfn3, Lrfn4, Lrfn5), and homologues in other vertebrates. Lrfn proteins share leucine-rich repeat (LRR)–immunoglobulin-like (Ig)–fibronectin type III (Fn)–transmembrane domain structure, which is also found in LRR–Ig–Fn superfamily proteins. Mouse Lrfn genes were expressed at adult stage predominantly in the brain. In the course of development, expression of Lrfn1, Lrfn3, and Lrfn4 started from immature neural cells, whereas that of Lrfn2 and Lrfn5 was limited to mature ones. Lrfn1–5 commonly encode glycoproteins spanning the plasma membrane, with their N-terminus located on the extracellular side. C-termini of Lrfn1, Lrfn2 and Lrfn4 were bound by PDZ domains of postsynaptic protein PSD95, re-distributing PSD95 to cell periphery where the Lrfn proteins were detected. These results suggest that Lrfn proteins are neuronal components with a role in the developing or mature vertebrate nervous system.