Crystal structure of GumB, an outer membrane lipoprotein essential for secretion of xanthan

JACOBS M, BIANCO MI, SALINAS SR, IELPI L.

Puerto Varas

Congreso; XII PABMB Congress; 2013

SOCIEDAD DE BIOQUÍMICA Y BIOLOGÍA MOLECULAR DE CHILE

Xanthan, an extracellular polysaccharide secreted by Xanthomonas campestris, is involved in several cellular processes and has a broad range of applications in industry. Here we report the 2.54 Å resolution structure of the GumB tetramer, an outer membrane lipoprotein encoded in the gum operon, which is essential for the secretion of xanthan. Overexpression, purification and crystallization of GumB were achieved replacing its signal peptide by a His6-tag. GumB structure shows features also presented in other membrane proteins involved in polysaccharide secretion. Each protomer of GumB has two domains, both of them showing structural homology with the periplasmic domains D1 and D2 of Wza, an outer membrane lipoprotein involved in the secretion of group I capsular polysaccharide in E. coli. The RMSD value between D1 domains of both proteins (residues 44-119 and 89-169 of GumB and Wza, respectively) was of 1.7452 Å and between D2 domains (residues 120-204 and 171-253 of GumB and Wza, respectively) was 1.2151 Å. Moreover, GumB tetramer has an electropositive region due to the presence of lysines and arginines which could interact with the negatively charged xanthan. This characteristic has also been observed in AlgE, a β-barrel outer membrane protein which possesses an electropositive pore to allow secretion of alginate.