cDNA cloning, structural features, and eucaryotic expression of human TAG-1/axonin-1

Axonal surface glycoproteins, composed of repeated immunoglobulin-like and fibronectin-type-III(FNIII)-like domains, mediate adhesion between axons or between axons and non-neuronal cells or extracellular matrix proteins. Several representatives of this group promote neurite outgrowth, when presented as substratum to neurons in culture, and have been implicated in axonal guidance mechanisms. TAG-1 and axonin-1 are presumptive species homologues of the rat and the chick, respectively; together with F11/F3, they form a subgroup of Ig/FNIII-like molecules containing a glycosyl-PtdIns membrane anchor. Recent reports on tumor suppressor genes encoding Ig-like and FNIII-like sequences prompted us to isolate the human homologue to TAG-1 and axonin-1. Polymerase chain reaction (PCR) primers were designed to regions conserved in both TAG-1 and axonin-1 using deoxyinosine at ambiguous positions. An expected 1000-bp fragment was obtained from cDNA derived from adult human cerebellum. Using this PCR fragment as a probe, several clones were isolated from a human fetal brain cDNA library. Nucleotide sequence analysis of a full-length clone, as expected, revealed a high degree of similarity to rat TAG-1 (91% identity) and chicken axonin-1 (75% identity) at the amino acid level. The encoded protein was then transiently expressed in monkey COS1 cells, and a stable mouse myeloma cell line was established expressing human TAG-1/axonin-1. The transfected COS1 and myeloma cells showed immunoreactivity on the cell surface with polyclonal anti-(chicken axonin-1) serum. On Western blots, the same antibodies recognized the recombinant protein migrating slightly slower on SDS/PAGE than chicken axonin-1. A comparison of chicken and human brain-tissue proteins by Western-blot analysis revealed a similar apparent molecular mass difference between the two species, which might be due to three additional N-glycosylation sites present on human TAG-1/axonin-1. Immunostaining of cryostat sections of embryonic retinas with polyclonal anti-(axonin-1) serum showed similar expression patterns in chicken and human samples at corresponding developmental stages. An additional shared feature of human TAG-1/axonin-1, rat TAG-1 and chick axonin-1 is their attachment to the cell membrane with a glycosyl-PtdIns anchor.