Catalyzes the synthesis of lactosylceramide (LacCer) via the transfer of galactose from UDP-galactose to glucosylceramide (GlcCer) (By similarity). LacCer is the starting point in the biosynthesis of all gangliosides (membrane-bound glycosphingolipids) which play pivotal roles in the CNS including neuronal maturation and axonal and myelin formation (By similarity). Plays a role in the glycosylation of BMPR1A and regulation of its protein stability (By similarity). Essential for extraembryonic development during early embryogenesis (By similarity). (Microbial infection) May play a role in the glycosylation of porcine reproductive and respiratory syndrome virus GP5 protein and may be involved in the regulation of viral proliferation. a beta-D-glucosyl-(1<->1')-N-acylsphing-4-enine + UDP-alpha-D-galactose = a beta-D-Gal-(1->4)-beta-D-Glc-(1<->1)-Cer(d18:1(4E)) + H(+) + UDP Protein modification; protein glycosylation. Sphingolipid metabolism. (Microbial infection) Interacts with porcine reproductive and respiratory syndrome virus GP5. Trans cisternae of Golgi stack. (Microbial infection) Up-regulated in response to porcine reproductive and respiratory syndrome viral infection. Belongs to the glycosyltransferase 7 family.