Ceramide synthase that catalyzes the transfer of the acyl chain from acyl-CoA to a sphingoid base, with high selectivity toward very- and ultra-long-chain fatty acyl-CoA (chain length greater than C22) (PubMed:16753040, PubMed:18541923, PubMed:22038835). N-acylates sphinganine and sphingosine bases to form dihydroceramides and ceramides in de novo synthesis and salvage pathways, respectively (PubMed:16753040, PubMed:18541923, PubMed:22038835). It is crucial for the synthesis of ultra-long-chain ceramides in the epidermis, to maintain epidermal lipid homeostasis and terminal differentiation (PubMed:22038835). a sphingoid base + a very long-chain fatty acyl-CoA = an N-(very-long-chain fatty acyl)-sphingoid base + CoA + H(+) docosanoyl-CoA + sphinganine = CoA + H(+) + N-docosanoylsphinganine sphinganine + tetracosanoyl-CoA = CoA + H(+) + N-tetracosanoylsphinganine hexacosanoyl-CoA + sphinganine = CoA + H(+) + N-hexacosanoylsphinganine 2-hydroxydocosanoyl-CoA + sphinganine = CoA + H(+) + N-(2-hydroxydocosanoyl)-sphinganine 2-hydroxytetracosanoyl-CoA + sphinganine = CoA + H(+) + N-(2-hydroxytetracosanoyl)-sphinganine a sphingoid base + an ultra-long-chain fatty acyl-CoA = an N-(ultra-long-chain-acyl)-sphingoid base + CoA + H(+) octacosanoyl-CoA + sphinganine = CoA + H(+) + N-(octacosanoyl)-sphinganine a fatty acyl-CoA + sphing-4-enine = an N-acylsphing-4-enine + CoA + H(+) octadecanoyl-CoA + sphinganine = CoA + H(+) + N-(octadecanoyl)-sphinganine 2-hydroxyoctadecanoyl-CoA + sphinganine = CoA + H(+) + N-(2-hydroxyoctadecanoyl)-sphinganine Lipid metabolism; sphingolipid metabolism. Predominantly expressed in testis (PubMed:16753040). In skin, present in the upper stratum spinosum and stratum granulosum (at protein level) (PubMed:22038835). Newborn mice die shortly after birth from transepidermal water loss (PubMed:22038835). Defects are due to a complete loss of ultra long chain (more than C26:0) ceramides (PubMed:22038835). Newborn skin appears unwrinkled, erythematous and sticky (PubMed:22038835). Skin is also prone to C.albicans infection (PubMed:22038835). Contains a predicted homeobox domain which is degenerated, lacking residues important for DNA-binding. Moreover, the protein localizes in the endoplasmic reticulum and not in the nucleus, which also argues against homeobox function (PubMed:22038835).