1.H.1.5.1
β-type IP39 protein of 264 aas and 4 TMSs in a 1 3 TMS arrangement.  Euglenoid flagellates have striped surface structures comprising pellicles, which allow the cell shape to vary from rigid to flexible during the characteristic movement of the flagellates. In Euglena gracilis, the pellicular strip membranes are covered with paracrystalline arrays of a major integral membrane protein, IP39, a four TMS protein with the conserved sequence motif of the PMP-22/EMP/MP20/Claudin superfamily. Suzuki et al. 2013 reported the three-dimensional structure of Euglena IP39 determined by electron crystallography. Two-dimensional crystals of IP39 formed a striated pattern of antiparallel double-rows in which trimeric IP39 units are longitudinally polymerised, resulting in continuously extending zigzag-shaped lines. Structural analysis revealed an asymmetric molecular arrangement in the trimer, suggesting that at least four different interactions between neighbouring protomers are involved. A combination of such multiple interactions would be important for linear strand formation of membrane proteins in a lipid bilayer (Suzuki et al. 2013).