Table 3 Key amino acids for intra-, trans- or cis interaction of classic CLDNs

W30 and G48/L49/W50 and R81

The canonical CLDN signature to create its first ECL structure.

[138]

S69/Q78/R81 and E159

Cis-interaction by a stable hydrogen bond between S69 and E159.

[140, 150]

F139

It interacts with W30 and stabilizes the secondary structure. Only CLDN-5 has F at this position among classic CLDNs.

[146]

L73 and F147/Y148

Cis-interaction. F147/Y148 creates a hydrophobic pocket.

[140]

H61 to K65

The interface of cis-interaction for dimerization.

[229]

P135

It makes TJ strands more rigid by reducing the conformational flexibility of cis-/trans-interaction

[141]

F35, N39 to Q44

The interface of trans-interaction. The interaction of these hydrophobic residues may create a barrier against water molecules.

[148, 150]

F147 to Q156

The interface of trans-interaction. F147A, Y148A or Q156E mutant does not form trans-interaction.

[230, 231]

K65 and D68

The salt bridge (or hydrogen bond) between these positions limits the ion permeability.

[146, 232]

Q57

A polar but uncharged amino acid like Q and H is necessary for some CLDNs to form cis-interaction and to localize at the TJs.

[161, 162]

Q57 and H61 and Q63

Putative ion gate for extracellular ions is located here in channel-forming CLDNs. Barrier forming CLDNs have Q57 and Q61 (only CLDN-5 has H here) and Q63 and make multiple hydrogen bonds to close this gate.

[148]