A Glucan Synthase-Remodeler Module Organizes Branched Glucan Assembly in the Fungal Cell Wall

The fungal cell wall is an essential extracellular matrix that underpins growth, morphogenesis, and pathogenesis, and its assembly requires the coordinated action of poorly understood enzyme networks. In Schizosaccharomyces pombe, we find that Ghs2, a glycoside hydrolase 16 (GH16) domain containing protein, localizes and functions in strict partnership with the {beta}-1,3-glucan synthase Bgs3 at sites of polarized growth. Ghs2 and Bgs3 physically associate and structural models position the Ghs2 catalytic domain proximal to the Bgs3 glucan extrusion pore. Solid-state NMR analyses show that Ghs2 and Bgs3 are required for {beta}-1,6-glucan production, and pharmacological and genetic evidence suggests that Ghs2 acts directly on nascent Bgs3-produced {beta}-1,3-glucan to generate {beta}-1,6-linked branch points. Together, our findings provide the first example of a glucan synthase physically coupled to a remodeling enzyme for branched glucan generation. Further we establish a new principle of fungal cell wall assembly in which synthase-modifier modules operate as inseparable units.