A secreted β-glucan-branching enzyme fromCandida albicans

Abstract

A M$_{\text{r}}$ 34000 wall protein was isolated as a by-product of the purification of an endo-(1-3)-$\beta $-glucanase from the culture filtrate of Candida albicans. The purified fraction contained no exo- or endo-$\beta $-glucanase activity, and analysis by SDS poly-acrylamide gel electrophoresis (SDS-PAGE) showed one protein band at M$_{\text{r}}$ 34000. Analysis by gel filtration high performance liquid chromatography (HPLC) of reaction products from incubations of the protein fraction with laminarioligosaccharides of five glucosyl units or greater revealed a unique glucanosyl transferase activity. The enzyme specifically cleaved laminaribiaose (G$_{2}$) from the reducing-end of a linear $\beta $-(1-3)-glucan and transferred the remainder to another laminari-oligosaccharide. The reaction with laminaripentaose (G$_{5}$) produced G$_{2}$ and a product eluting at the position of G$_{8}$. Analysis of the latter transferase product by $^{13}$C-and $^{1}$H-nuclear magnetic resonance (NMR) spectroscopy shows it to be a branched molecule containing a $\beta $-(1-3)-$\beta $-(1-6)-branchpoint. It is suggested that the M$_{\text{r}}$ 34000 wall protein is a glucan branching enzyme, perhaps the key enzyme responsible for the transformation of the initial linear $\beta $-1-3)-glucan into the branched $\beta $-(1-3)-$\beta $-1-6)-glucan as found in the cell wall of C. albicans.