Working model. (A) In WT salivary gland cells, EE–TGN retrograde trafficking mediated by Past1, PI4KII, and Syx16 is required for normal SG maturation. PI4P is observed on both EEs and SGs. CD63 also reaches apical membrane through constitutive secretion. (B) Loss of PI4KII results in formation of enlarged EEs, lack of endosomal tubules, and failure of SG maturation. PI4P also fails to accumulate on SG membranes due to the loss of PI4KII-mediated retrograde trafficking. (C) Loss of Past1 or knockdown of Syx16 results in formation of enlarged EEs and failure of SG maturation due to defects in PI4KII-dependent endosomal tubule formation and retrograde trafficking. As a result, PI4P levels on SGs are reduced and PI4P accumulates on EEs.