HRG1 is a family of conserved, membrane-bound permeases that reside in distinct intracellular compartments and bind and transport haem in metazoa [1]. These proteins carry four transmembrane domains, 4xTMs, modelled here in two pairs, the two N-terminal and the two more C-terminal.

1: Haem homeostasis is regulated by the conserved and concerted functions of HRG-1 proteins. Rajagopal A, Rao AU, Amigo J, Tian M, Upadhyay SK, Hall C, Uhm S, Mathew MK, Fleming MD, Paw BH, Krause M, Hamza I; Nature. 2008;453:1127-1131. PMID:18418376