PANTHER database (Mi et al., 2005) was used to determine the protein class distribution of annotated transcripts and protein sequences in primary Tupaia hepatocytes (PTHs) generated in this study and transcriptome from primary human hepatocytes (PHHs) reported by Hart et al. (2010). Photo-cross-linking with peptide ligand and tandem purification of the target molecule(s) L-photo-leucine-bearing wild-type bait peptide (WT b ) or control bait peptide (N9K b ) was dissolved in DMSO in dark. L-photo-leucine contains a photoactivatable diazirine ring, irradiation of UV light at 365 nm induces a loss of nitrogen of the diazirine ring, and yields a reactive carbene group with short half-life for covalent cross-linking at nearly zero distance. For tandem purification, WT b or N9K b at indicated concentrations was applied to ∼1 × 10 7 hepatocytes plated on collagen-coated dishes. Cells were cross-linked by UV irradiation and then washed to remove residual free peptides and subsequently lysed with 1 ml radioimmunoprecipitation assay (RIPA, pH 7.4) buffer containing 20 mM Tris, 150 mM NaCl, 0.1% SDS; 0.5% sodium deoxycholate, 1% NP40, and 1× protease inhibitor cocktail (Roche). The cell lysates were precipitated with 100 μl streptavidin T1 magnetic beads, eluted with 50 μl nonreducing SDS-PAGE loading buffer and then diluted with cold RIPA buffer to a final volume of 1 ml and was precipitated with 100 μl (1 × 10 8 ) 2D3-conjugated M-270 dynabeads, then eluted with 100 μl nonreducing loading buffer. The elute, with or without PNGase F treatment, was diluted to 1 ml with RIPA buffer and was precipitated again with 100 μl streptavidin T1 magnetic beads, followed by extensive washing, and finally eluted by boiling 5 min with 20 μl SDS-PAGE loading buffer. The samples were then analyzed with 12% SDS-PAGE and silver staining. For photocross-linking analysis of primary cells, cell lines, or NTCP-or control plasmid-transfected Huh-7 or 293T cells, WT b or N9K b bait peptide in the presence or absence of competing peptide was applied to 2 × 10 6 cells, photo-cross-linking was conducted similarly as described above. The cross-linked samples were precipitated with streptavidin T1 magnetic beads and separated by SDS-PAGE, and followed by Western blotting with mAb 2D3 (recognizing bait peptides) or mAb 1D4 against C-terminal tag C9.