Catalyzes the formation of branch points in alpha-glucans by cleavage of an alpha-1,4 glycosidic bond and subsequent transfer of the cleaved-off oligosaccharide to a new alpha-1,6 position (Probable). Is probably involved in the biosynthesis of 6-O-methylglucosyl lipopolysaccharides (MGLP). Transfers a segment of a (1->4)-alpha-D-glucan chain to a primary hydroxy group in a similar glucan chain. Belongs to the glycosyl hydrolase 57 family.