Catalytic subunit of the 1,3-beta-glucan synthase. Synthesizes 1,3-beta-glucan, a major structural component of the cell wall. Involved in cell wall synthesis, maintenance and cell wall remodeling (By similarity). [(1->3)-beta-D-glucosyl](n) + UDP-alpha-D-glucose = [(1->3)-beta-D-glucosyl](n+1) + H(+) + UDP Component of the 1,3-beta-glucan synthase, composed of at least catalytic subunit fksA, and a regulatory subunit. Belongs to the glycosyltransferase 48 family.