Fig. 3: The G′-portal is a unique feature of CD1c.

a Details at the G′-portal of CD1c showing the protrusion of the GD3 headgroup (yellow ball-and-stick). Amino acid side chains are depicted in space-filling format. The G′-portal is constituted by nine amino acids, including four large residues His84, Gln87, Tyr89, and Lys91, and five smaller residues that create the accessible entry point, i.e. Ser90, Leu140, Ser139, Ser143, and Leu147. b Structural alignment of CD1c-GD3 (teal) versus CD1c-mock (pink) showing the movements of the α-helices, with arrows depicting direction of motion from the CD1c-mock to CD1c-GD3 structures. Cα atoms of Asp86 and Leu147 are depicted in spheres with the corresponding colours. Lipids are depicted in ball-and-stick, with GD3 coloured in yellow and spacer lipids in CD1c-mock coloured in white. c Comparison of the G′-portal in the structures of CD1c-mock versus CD1c-GD3. The major axis is measured from Lys91 Cβ to Leu147 Cγ, and the minor axis is measured from His84 Cβ to Leu140 Cγ. d Side view to the G′-portal of CD1c in comparison with the CD1a (brown), CD1b (green) and CD1d (blue), represented as surfaces. The blocking residues of the corresponding G′-portal position in each CD1 isoform are individually coloured as footprint. e Top view to the F′-portal of all CD1 isoforms demonstrating the sideways presentation of CD1c while the same GD3 lipid (yellow spheres) is presented upright in the other isoforms. CD1d-GD3 was reported previously (PDB ID 3AU1)²⁸. f Comparison of the CD1c G′-portal versus the open end of MHC II. CD1c and GD3 are in the same colour code as previous panels. The presented peptide in MHC II is depicted in green-cyan ball-and-sticks. Right, Equivalent position of the G′-portal in CD1c and the open end of MHC II. The structures are portrayed in surface representation. The structure of MHC II-peptide without superposition of CD1c-GD3 is placed at the right for reference. MHC II-peptide was reported previously (PDB ID 7T6I)⁵².