I am 外行人， too. Let me try to convince you.
How did they pick NTCP? They incubated cells with a high concentration (200
nM) of viral protein baits, photo-cross-linked, and immunoprecipitated the
protein complexes. They saw a thick band at 60 kDa. They ran another gel and
cut the band at that position and did MS. NTCP showed up. They believed
that that was the one.
Here are the problems. (1) With a high concentration of baits and cross-
linking, you would pull down a lot of proteins. They admitted that there
were smears. The thick bands may represent the real receptor or the abundant
protein on the cell surface. It turned out the latter is possible. NTCP is
a liver-specific protein and very abundant on the surface of hepatocytes.
(2) After they saw NTCP in the MS assay, they should've gone back to strip
those blots and reprobed with an antibody against NTCP to confirm that that
protein was present in the complex. The reasons why they did not do it are
an open question.
(3) With little doubt, there was more than one type of protein in the gel
slices that they cut and ran MS. How did they determine that NTCP was the
candidate for further examinations? They did not give a rationale or
evidence. The logic guess is that NTCP is a liver protein,membrane protein,
and fits well with the idea of being a receptor.
The later attempts to confirm that NTCP is the receptor are not convincing
and were done using wrong methods.
