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. Author manuscript; available in PMC: 2014 Dec 1.
Published in final edited form as: Exp Eye Res. 2013 Jun 12;117:10.1016/j.exer.2013.05.020. doi: 10.1016/j.exer.2013.05.020

Fig. 1.

Fig. 1

Linear diagram of secreted lacritin. PSIPRED (v3.3) predicted α-helices are indicated by rectangles; the arrow indicates a short predicted β-strand. The most C-terminal α-helix is amphipathic and targets the cell surface proteoglycan syndecan-1 after heparanase modification (Wang et al., 2006). Both the hydrophobic face (L108/L109/F112) and the cationic face (Q103/K107 and K111) of lacritin are involved (Zhang et al., 2013). These target three syndecan-1 elements (Zhang et al., 2013): (i) the conserved hydrophobic sequence GAGAL, and (ii) chondroitin-4-sulfated and (iii) heparanase-cleaved 3-O-sulfated heparan sulfated chains on N-terminal serines 15, 23 and 25 (human syndecan-1 numbering excludes the signal peptide). α-helicity of lacritin’s C-terminal a-helix has been validated by circular dichroism (Wang et al., 2006; Zhang et al., ’13). Syndecan-1 has a short transmembrane domain known only for cytoskeletal signaling. Rapid lacritin signaling appears to be attributable to an associated G-protein coupled receptor, as first implied by pertussis toxin inhibitable mitogenic signaling (Wang et al., 2006).