9.B.92 The Folate Receptor (FR) Family

Folate receptor proteins have been implicated in folate transport (Yasuda et al., 2008). They are homologous to riboflavin binding proteins (e.g., BAA08709). FRα of humans is 257 aas long with N- and C-terminal TMSs. Retbindins (EDC10974; 247aas) are also in this family.



Foraker, A.B., C.M. Khantwal, and P.W. Swaan. (2003). Current perspectives on the cellular uptake and trafficking of riboflavin. Adv Drug Deliv Rev 55: 1467-1483.

Jones, S.K., A. Sarkar, D.P. Feldmann, P. Hoffmann, and O.M. Merkel. (2017). Revisiting the value of competition assays in folate receptor-mediated drug delivery. Biomaterials 138: 35-45.

Kelley, R.A., M.R. Al-Ubaidi, and M.I. Naash. (2015). Retbindin is an extracellular riboflavin-binding protein found at the photoreceptor/retinal pigment epithelium interface. J. Biol. Chem. 290: 5041-5052.

Sabetian, S., M.S. Shamsir, and M. Abu Naser. (2014). Functional features and protein network of human sperm-egg interaction. Syst Biol Reprod Med 60: 329-337.

White, H.B., 3rd and A.H. Merrill, Jr. (1988). Riboflavin-binding proteins. Annu. Rev. Nutr. 8: 279-299.

Yasuda, S., S. Hasui, M. Kobayashi, S. Itagaki, T. Hirano, and K. Iseki. (2008). The mechanism of carrier-mediated transport of folates in BeWo cells: the involvement of heme carrier protein 1 in placental folate transport. Biosci. Biotechnol. Biochem. 72: 329-334.


TC#NameOrganismal TypeExample

Folate binding receptor, FRα (Jones et al. 2017).


FRα of Homo sapiens (P15328)


IZUMO1R, FolR4, JUNO of 250 aas.  Receptor for IZUMO1 present at the cell surface of oocytes (oolemma), which is essential for gamete recognition and fertilization. The IZUMO1:IZUMO1R/JUNO interaction is a necessary adhesion event between sperm and egg that is required for fertilization but is not sufficient for cell fusion. The ligand-receptor interaction probably does not act as a membrane 'fusogen' (Sabetian et al. 2014). Does not bind folate.


JUNO of Homo sapiens


Riboflavin binding protein, RBP, of 242 aas and 1 N-terminal TMS.  It is believed to play a role in riboflavin transport (White and Merrill 1988; Foraker et al. 2003).

RBP of Pelodiscus sinensis japonicus (Chinese soft-shelled turtle)


Uncharacterized protein of 265 aas

UP of Branchiostoma floridae (Florida lancelet) (Amphioxus)


TC#NameOrganismal TypeExample

Retbindin of 229 aas and 1 N-terminal TMS. Riboflavin-binding protein that may play a role in retinal flavin transport. It is an extracellular riboflavin binding protein found in the photoreceptor/retinal pigment epithelium interface (Kelley et al. 2015).

Retbindin of Homo sapiens


Retbindin of 323 aas and 1 or 2 TMSs, RTBDN.

Retbindin of Monodelphis domestica (Gray short-tailed opossum)