cDNA cloning and sequencing reveals human tear prealbumin to be a member of the lipophilic-ligand carrier protein superfamily.

The gene encoding human tear prealbumin, a major component of the protein fraction of tear fluid, was cloned from total cDNA of lacrimal gland by polymerase chain reaction using synthetic oligonucleotides derived from N-terminal amino acid sequences of the purified protein. Sequence analysis and a computer-assisted homology search revealed this protein to be a member of the lipocalin superfamily, consisting of hydrophobic-ligand carriers. The deduced amino acid sequence of tear prealbumin shares 58% identity with von Ebner's gland protein from rat, which is supposed to be involved in taste reception. In addition, significant homology has also been found with other members of the lipocalins, e.g. with beta-lactoglobulin. The predicted secondary structure of tear prealbumin resembles that of beta-lactoglobulin in the number and positions of nine beta-sheets and one alpha-helix at the C-terminal part of the protein, thus indicating a three-dimensional structure similar to beta-lactoglobulin. Protein sequencing revealed that the observed electrophoretic heterogeneity of tear prealbumin is due to subtle differences at the N terminus of the protein. The function of tear prealbumin as a lipophilic carrier was further supported by the fact that it binds [3H]retinol in vitro. Although this protein was originally described to be tear-specific, a tear prealbumin-specific antiserum also reacted with proteins of human saliva, sweat, and nasal mucus.