Alpha-2,6-sialyltransferase involved in the synthesis of alpha-series gangliosides. Has activity toward GD1a, GT1b and GM1b. Has no activity toward glycoproteins. Responsible for the biosynthesis of DSGG (disialylgalactosylgloboside) from MSGG (monosialylgalactosylgloboside) in normal and malignant kidney. Participates in the synthesis of disialyl Lewis a (Le(a)).