2.A.94 The Phosphate Permease (Pho1) Family
Pho1 of A. thaliana (TC# 2.A.94.1.1) is a member of the PHO1 family (11 paralogues in A. thaliana). It functions in inorganic phosphate transport and homeostasis. Pho1 catalyzes efflux of phosphate from epidermal and cortical cells into the xylem (Quaghebeur and Rengel, 2004; Stefanovic et al., 2007). The SPX superfamily domain is an N-terminal soluble domain. These proteins belong to the EXS (Erd1/Xpr1/Syg1) superfamily.
Erd1 (TC# 2.A.94.1.7) is a member of the Pho1 family and is of 362 aas possibly with 7 TMSs in a 1 + 2 + 3 + 1 TMS arrangement, where the first 3 TMSs are stronly hydrophobic, but the last 4 are only moderately hydrophobic. It is required for the retention of luminal endoplasmic reticulum/Golgi proteins and affects glycoprotein processing in the Golgi apparatus. Protein glycosylation in the Golgi is a sequential process that requires proper distribution of transmembrane glycosyltransferases in the appropriate Golgi compartments. Some of the cytosolic machinery required for the steady-state localization of some Golgi enzymes is known. Erd1 facilitates Golgi glycosyltransferase recycling by directly interacting with both the Golgi enzymes and the cytosolic receptor, Vps74 (Q06385) (Sardana et al. 2021). Loss of Erd1 function results in mislocalization of Golgi enzymes to the vacuole/lysosome. Thus, Erd1 forms an integral part of the recycling machinery and ensures productive recycling of several early Golgi enzymes as well as how the localization of Golgi glycosyltransferases is spatially and temporally regulated.
The generalized reaction catalyzed by Pho1 is:
Pi (cells) → Pi (xylem)
This family belongs to the IT Superfamily.