Structural characterisation of a MAPR‐related archaeal cytochrome b_5M protein

We recently reported that the membrane‐associated progesterone receptor (MAPR) protein family (mammalian members: PGRMC1, PGRMC2, NEUFC and NENF) originated from a new class of prokaryotic cytochrome b 5 (cytb 5 ) domain proteins, called cytb 5 M ( M APR‐like). Relative to classical cytb 5 proteins, MAPR and ctyb 5M proteins shared unique sequence elements and a distinct heme‐binding orientation at an approximately 90° rotation relative to classical cytb 5 , as demonstrated in the archetypal crystal structure of a cytb 5M protein (PDB accession number 6NZX ). Here, we present the crystal structure of an archaeal cytb 5M domain ( Methanococcoides burtonii WP_011499504.1 , PDB: 6VZ6 ). It exhibits similar heme binding to the 6NZX cytb 5M , supporting the deduction that MAPR‐like heme orientation was inherited from the prokaryotic ancestor of the original eukaryotic MAPR gene.