Cellulose synthase operon, protein C of 1157 aas and up to 18 C-terminal (720 - 1157 aas) β-strands with a single N-terminal α-TMS, BcsC or YhjL (Zogaj et al. 2001). Translocation across the outer membrane occurs through the BcsC porin, which extends into the periplasm via 19 tetra-tricopeptide repeats (TPR). Acheson et al. 2019 presented the crystal structure of a truncated BcsC, encompassing the last TPR repeat and the complete outer membrane channel domain, revealing a 16-stranded, β barrel pore architecture. The pore is blocked by an extracellular gating loop, while the extended C terminus inserts deeply into the channel and positions a conserved Trp residue near its extracellular exit. The channel is lined with hydrophilic and aromatic residues suggesting a mechanism for facilitated cellulose diffusion based on aromatic stacking and hydrogen bonding (Acheson et al. 2019).