Protein folding in the endoplasmic reticulum (ER) depends on Ca 2+ ; uptake of Ca 2+ into the ER is mediated by SERCA3. The 5′-flanking region of the SERCA3 gene (ATP2A3) contains numerous binding sites for the transcription factors Sp1 and Sp3. Biotin affects the nuclear abundance of Sp1 and Sp3, which may act as transcriptional activators or repressors. Here we determined whether biotin affects the expression of the SERCA3 gene and, thus, protein folding in human lymphoid cells. Jurkat cells were cultured in media containing 0.025 nmol/L biotin (denoted "deficient") or 10 nmol/L biotin ("supplemented"). The transcriptional activity of the full-length human SERCA3 promoter was 50% lower in biotin-supplemented cells compared to biotin-deficient cells. Biotin-dependent repressors bind to elements located 731 to 1312 basepairs upstream from the transcription start site in the SERCA3 gene. The following lines of evidence suggest that low expression of SERCA3 in biotinsupplemented cells impaired folding of secretory proteins in the ER, triggering Unfolded Protein Response: (i) sequestration of Ca 2+ in the ER decreased by 14-24% in response to biotin supplementation; (ii) secretion of interleukin-2 into the extracellular space decreased by 75% in response to biotin supplementation; (iii) the nuclear abundance of stress-induced transcription factors increased in response to biotin supplementation; and (iv) the abundance of stress-related proteins such UBE1, GADD153, XBP1, and phosphorylated eIF2α increased in response to biotin supplementation. Collectively, this study suggests that supplements containing pharmacological doses of biotin may cause cell stress by impairing protein folding in the ER.