8.A.74 The TM9 or Phg1 Targeting Receptor (Ppg1) Family
TM9 family proteins (also named Phg1 proteins) control cell adhesion by determining the cell surface localization of adhesion proteins such as the Dictyostelium SibA protein. Perrin et al. 2015 showed that the glycine-rich transmembrane segments (TMSs) of SibA is sufficient to confer Phg1A-dependent surface targeting to a reporter protein. In Dictyostelium phg1A- knockout (KO) cells, proteins with glycine-rich TMSs were less efficiently transported out of the endoplasmic reticulum (ER) and to the cell surface. Phg1A, as well as its human ortholog TM9SF4 specifically associated with glycine-rich TMSs. In human cells, genetic inactivation of TM9SF4 resulted in an increased retention of glycine-rich TMSs in the endoplasmic reticulum, whereas TM9SF4 overexpression enhanced their surface localization. The bulk of the TM9SF4 protein was localized in the Golgi complex, and a proximity-ligation assay suggested that it might interact with glycine-rich TMSs. It thus appears that one of the main roles of TM9 proteins is to serve as intramembrane cargo receptors controlling exocytosis and surface localization of a subset of membrane proteins.