4.D.1.1.14
GtrB of 318 aas and 2 TMSs.  The 3.0 Å resolution crystal structure of GtrB, a glucose-specific polyisoprenyl-glycosyltransferase (PI-GT) from Synechocystis, has been reported showing a tetramer in which each protomer contributes two helices to a membrane-spanning bundle (Ardiccioni et al. 2016). The active site is 15 Å from the membrane, raising the question of how water-soluble and membrane-embedded substrates are brought into apposition for catalysis. A conserved juxtamembrane domain harbours disease mutations, which compromised activity in GtrB in vitro and in human DPM1 tested in zebrafish.  A role of this domain in shielding the polyisoprenyl-phosphate for transport to the active site has been proposed (Ardiccioni et al. 2016).