OBCAM (Opioid-binding cell adhesion molecule, also known as OPCML) is a member of the IgLON family of cell adhesion molecules. All IgLONs are GPI-linked glycoproteins, contain three C2 type Ig-like domains, and are expressed in cerebral cortex and hippocampus (1, 2). The name IgLON derives from family membership in the Ig superfamily, and the first letters of the names of group’s molecules; LAMP,OBCAM, and Neurotrimin. Recently, membership in the group has been expanded by one with the addition of Kilon (Kindred of IgLON), and members of this group are now often referred to Diglons, based on the dimerizing nature of the IgLONs (1, 2). Human OBCAM is synthesized as a 345 amino acid (aa) preproprecursor that contains a 27 aa signal sequence, a 295 aa mature region, and a C‑terminal 23 aa prosegment (3). The prosegment is cleaved to generate the GPI-link. OBCAM varies in molecular weight, ranging from 46 kDa to 65 kDa (4‑6). The difference is not due to alternate splicing but to differential glycosylation (6). Although it is not unusual for GPI-linked proteins to be solubilized, to date there is no evidence that OBCAM functions as a soluble molecule (1). Mature human OBCAM is 98%, 99%, and 98% aa identical to mature bovine, rat and mouse OBCAM, respectively. OBCAM has limited expression, occurring principally in telencephalon and ovarian epithelium (7, 8). In brain, it is found associated with dendrites and post-synaptic membranes, where it may maintain synaptic architecture (1, 5). In ovary, it has been suggested to be a tumor-suppressor factor (8). The receptor(s) for OBCAM appears to be other members of the IgLON family, and a dimer is the functional unit. While neurotrimin appears to function as both a homodimer and heterodimer, all other family members (including OBCAM) show a preference for heterodimerization. OBCAM forms strong trans (between cells) heterodimers with LAMP, and modest heterodimers with Neurotrimin. There is hardly any binding with itself. Kilon likely binds OBCAM, but this interaction is not well studied (1). OBCAM heterodimers apparently bind to almost all possible IgLON heterodimer combinations on other cells. In cis (same cell), OBCAM also binds to LAMP and Neurotrimin (2).

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