Oligomerization, membrane anchoring, and cellulose-binding characteristics of AbpS, a receptor-like Streptomyces protein

Streptomyces reticuli produces a 34.6-kDa surface-anchored protein (AbpS) whose surface-exposed N terminus binds strongly to Avicel, a dominantly crystalline type of cellulose. The generation of a large set of mutated abpS-genes and the subsequent analysis of the corresponding proteins in vitro as well as in vivo in a Streptomyces host allow the assignment of the following characteristics for AbpS. (i) Amino acid residues participating directly in the cellulose-interaction are located at the N terminus. (ii) As ascertained by cross-linking experiments, AbpS forms homotetramers in its soluble as well as cellulose-bound form. (iii) The intermolecular assembly of four AbpS molecules is governed by two domains (including amino acids 60-110 and 161-212). Both domains possess large portions of alpha-helical regions in which hydrophobic amino acids are located on one side as known from coiled-coil proteins. (iv) The C-terminal part of AbpS comprising 35 amino acids contains a transmembrane domain. Due to the surface-exposed N terminus of AbpS and the presence of transmembrane helix the C terminus has to be situated in the cytoplasm of the S. reticuli hyphae. Thus AbpS connects the interior of the mycelia with the extracellular space and binds cellulose using a unique cellulose-binding module.