A highly hydrophobic protein with six transmembrane structure that is coded by the candidate tumor suppressor gene 101F6 located in the human chromosome 3p.21.3 and a possible member of the cytochrome b 561 protein family was expressed, purified, and characterized in its functional form for the first time. The protein was heterologously expressed in methylotrophic yeast Pichia pastoris as a fusion protein containing a C‐terminal thrombin‐specific sequence and an 8‐His residue tag. Purification was achieved by ion exchange chromatography on DEAE‐Sepharose and affinity chromatography on Ni‐NTA‐Sepharose. SDS‐PAGE analysis revealed a single protein band with an estimated molecular weight of 26 kDa, while Western blot and MALDI‐TOF‐MS analysis confirmed the presence of the cytochrome b561 specific sequence in the protein. The 101F6 protein was found to be reducible by ascorbate efficiently and to have two midpoint potentials at +89.5 and +13.1 mV, slightly lower than the corresponding values of +155 and +62 mV, respectively, of bovine adrenal cytochrome b 561, despite a lower conservation of the putative ascorbate binding site sequence in the 101F6 protein. The “modified motif 1” sequence unique in 101F6 protein may be responsible for other molecular functions, such as protein‐protein interactions, in the endoplasmic membranes.

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