Noncanonical Role of the PDZ4 Domain of the Adaptor Protein PDZK1 in the Regulation of the Hepatic High Density Lipoprotein Receptor Scavenger Receptor Class B, Type I (SR-BI)

Abstract The four PDZ (PDZ1-PDZ4) domain-containing-adaptor protein PDZK1 controls the expression, localization and function of the HDL receptor SR-BI in hepatocytes in vivo via a PDZ4-dependent mechanism involving the binding of SR-BI′s cytoplasmic carboxy terminus to the canonical peptide binding sites of the PDZ1 or PDZ3 domains (no binding to PDZ2 or PDZ4). Using transgenic mice expressing in the liver domain deletion (ΔPDZ2 or ΔPDZ3), domain replacement (PDZ2→1) or target peptide binding-negative (PDZ4[G389P]) mutants of PDZK1, we found that neither PDZ2 nor PDZ3, nor the canonical target peptide binding activity of PDZ4 were necessary for hepatic SR-BI regulatory activity. Immunohistochemical studies established that the localization of PDZK1 on hepatocyte cell-surface membranes in vivo is dependent on its PDZ4 domain and the presence of SR-BI. Analytical ultracentrifugation and hydrogen deuterium exchange mass spectrometry suggested that the requirement for PDZ4 for localization and SR-BI regulation is not due to PDZ4-mediated oligomerization or induction of conformational changes in the PDZ123 portion of PDZK1. However, Surface Plasmon Resonance analysis showed that PDZ4, but not the other PDZ domains, can bind vesicles that mimic the plasma membrane. Thus, PDZ4 may potentiate PDZK1′s regulation of SR-BI by promoting its lipid-mediated attachment to the cytoplasmic membrane. Our results show that not all of the PDZ domains of a multi-PDZ domain-containing adaptor protein are required for its biological activities and that both canonical target peptide binding and non-canonical (peptide binding independent) capacities of PDZ domains may be employed by a single such adaptor for optimal in vivo activity.