Caenorhabditis elegans UNC5 (UNC = behaviorally uncoordinated) and its mammalian homologues, UNC5H1-4, UNC5A-D, and rostral cerebellar malformation (RCM), are transmembrane proteins belonging to the immunoglobulin (Ig) superfamily. All UNC5 family members have two Ig and two thrombospondin type 1 domains in their extracellular regions, as well as a conserved ZU-5 domain, a DCC (Deleted in Colorectal Cancer)-binding domain (DB) and a C-terminal death domain (DD) in their cytoplasmic regions (1, 2). Human UNC5H4 cDNA encodes a 948 amino acid (aa) residues type I membrane protein with a putative 14 aa signal peptide and 359 aa extracellular domain. The extracellular domain of human UNC5H4 shares approximately 97% and 66% aa sequence identity with mouse UNC5H4 and human UNC5H3, respectively.
UNC5 family proteins are receptors for the netrin/UNC6 family of secreted axon guidance cues that are laminin-related proteins. Netrin family proteins can act as a chemoattractant for some axons and as a chemorepellent for others. Besides UNC5, netrin family proteins also bind to the DCC family of type I transmembrane receptors and to adenosine A2b receptor, a G protein-coupled seven-transmembrane receptor belonging to the adenosine receptor family (3, 4). In vitro, netrin binding to DCC family receptors in the absence of UNC5 is associated with axon attraction. However, the DCC-mediated attraction to netrin is converted to repulsion by binding of UNC5 to the DCC-netrin complex. In vivo, the mechanisms of netrin-dependent axon attraction and repulsion are more complex and may include UNC5-mediated repulsion that is independent of DCC (1, 5). Besides their roles in axon guidance and neuronal migration, the UNC5 and DCC families also act as dependence receptors and exert pro-apoptotic effects in the absence of netrin (6).
