Specific cargo receptor protein for the plasma membrane ATPase PMA1 that acts with PSG1 to promote the transport and maturation of PMA1 (PubMed:28727280). EXP1 and PSG1 probably act sequentially to promote PMA1 sorting between the ER and the Golgi, with EXP1 promoting PMA1 export from the ER to the Golgi while PSG1 has a role in PMA1 maturation or quality control in the Golgi (PubMed:28727280). Interacts with PMA1 and PSG1. Cycles between the ER and the Golgi apparatus. Lead to ER retention of PMA1. Present with 3260 molecules/cell in log phase SD medium.