DescriptionOrganic anion transporters (OATs), as one group of the important Drug/xenobitic transporters, play vital roles in the body disposition of environmental toxins and clinically anionic drugs. Numerous works have forged an extensive framework on OATs. However, more works are required to explore the functionally critical amino acid residues and motifs to glean comprehensive information on relationship between structure and function. In the first part of the thesis, we investigated the role of dileucine (L6L7) at the amino terminus of hOAT1 and GXXXG motifs in its transmembrane domains 2 (G144XXXG148) and 5 (G223XXXG227) of hOAT1 in the expression and function of the transporter by using mutants made by site-directed mutagenesis approach. Mutant transporter L6A/L7A, G144A and G148A showed no transport activity due to its complete loss of surface expression. Proteasomal inhibitor and lysosomal inhibitor treatment suggested the mutant L6A/L7A- G144A- and G148A transporters were degraded through proteasomal pathway. Treatment of L6A/L7A- expressing cells with two chemical chaperones could not repair the misfolding of the mutant transporter in Endoplasmic Reticulum. For mutant transporters G223A and G227A, only G227 showed dramatic reduced transport activity due to dramatic loss in expression. Proteasomal or lysosomal inhibitors resulted in partial recovery of total cell expression of the mutant G227A hOAT1, but not recovery of surface expression and function. Our data suggest that the L6L7 and GXXXG motifs in transmembrane domains 2 and 5 play critical roles in the stability of hOAT1. Our lab has reported that activation of protein kinase C (PKC) leads to accelerated internalization of hOAT1. However the underlying mechanism is still unclear. In the second part of the thesis, we indentified that ubiquitination of hOAT1 was significantly increased after PKC activation by phorbol 12-myristate 13-acetate (PMA) and Angiotensin II (AngII). And the PKC-induced ubiquitination of a mutant N5KR carrying multiple lysine substitutions was abolished. Importantly, cell surface biotinylation experiments showed that the N5KR mutant which has the minimal ubiquitination counteracted against the enhanced retrieval and accelerated degradation of surface hOAT1 proteins by PKC activation, which established a strong correlation of PKC-dependent endocytosis and PKC-dependent ubiquitination of hOAT1.