Participates in the initial step of the glucosylceramide-based glycosphingolipid/GSL synthetic pathway at the cytosolic surface of the Golgi. Catalyzes the transfer of glucose from UDP-glucose to ceramide to produce glucosylceramide/GlcCer (such as beta-D-glucosyl-(1<->1')-N-acylsphing-4-enine) (PubMed:10430909, PubMed:16109770, PubMed:28373486). Glucosylceramide is the core component of glycosphingolipids/GSLs, amphipathic molecules consisting of a ceramide lipid moiety embedded in the outer leaflet of the membrane, linked to one of hundreds of different externally oriented oligosaccharide structures (PubMed:10430909). Glycosphingolipids are essential components of membrane microdomains that mediate membrane trafficking and signal transduction (PubMed:10430909). They are implicated in many fundamental cellular processes, including growth, differentiation, migration, morphogenesis, cell-to-cell and cell-to-matrix interactions (PubMed:10430909). They are required for instance in the proper development and functioning of the nervous system (PubMed:16109770). As an example of their role in signal transduction, they regulate the leptin receptor/LEPR in the leptin-mediated signaling pathway (PubMed:23554574). They also play an important role in the establishment of the skin barrier regulating keratinocyte differentiation and the proper assembly of the cornified envelope (PubMed:17145749, PubMed:23748427). The biosynthesis of GSLs is also required for the proper intestinal endocytic uptake of nutritional lipids (PubMed:22851168). Catalyzes the synthesis of xylosylceramide/XylCer (such as beta-D-xylosyl-(1<->1')-N-acylsphing-4-enine) using UDP-Xyl as xylose donor (PubMed:33361282). an N-acylsphing-4-enine + UDP-alpha-D-glucose = a beta-D-glucosyl-(1<->1')-N-acylsphing-4-enine + H(+) + UDP an N-acylsphing-4-enine + UDP-alpha-D-xylose = a beta-D-xylosyl-(1<->1')-N-acylsphing-4-enine + H(+) + UDP N-(9Z-octadecenoyl)-sphing-4-enine + UDP-alpha-D-xylose = beta-D-xylosyl-(1<->1')-N-(9Z-octadecenoyl)-sphing-4-enine + H(+) + UDP Lipid metabolism; sphingolipid metabolism. Interacts with RTN1; regulates the ceramide glucosyltransferase activity of UGCG. Expressed early in preimplantation development, being already detected in eight-cell-stage embryos. The D1, D2, D3, (Q/R)XXRW motif is a critical part of the GCS active site, involved in catalysis and UDP-sugar binding. Embryonic lethal (PubMed:10430909). Homozygous knockout embryos initiate gastrulation with clear differentiation into embryonic germ layers, mesoderm, endoderm and ectoderm as well as the regional expression of critical genes (PubMed:10430909). However, a major apoptotic process leads to their resorption that starts at 7.5 dpc and is completed at 9.5 dpc (PubMed:10430909). Neural cell-specific conditional knockout does not affect early brain development but mice die between postnatal days 11 and 24 (PubMed:16109770). Dysfunction of cerebellum and peripheral nerves associated with some structural defects are observed (PubMed:16109770, PubMed:20544855). Peripheral nerves display increased surface area for both axon and myelin (PubMed:16109770). Purkinje cells undergo axonal degeneration associated with a disruption of myelin sheaths (PubMed:20544855). Forebrain neuron-specific conditional knockout leads to development of progressive obesity, hyperleptinemia, and glucose intolerance (PubMed:23554574). Epidermal-specific conditional knockout leads to a significant decrease of the total glucosylceramide content in the epidermis, a failure of the skin water barrier and a detachment of the stratum corneum (PubMed:17145749). Enterocyte-specific conditional knockout mice display deficient absorption of nutritional lipids (PubMed:22851168). Severe defects in intestinal epithelial differentiation also appear between postnatal days 5 and 7 but not before (PubMed:22851168). Hepatocyte-specific conditional knockout does not change basic liver functions with respect to sterol, glucose, and lipoprotein homeostasis (PubMed:20432257). Belongs to the glycosyltransferase 2 family.