In vitro and in vivo analysis of the binding of the C terminus of the HDL receptor scavenger receptor class B, type I (SR-BI), to the PDZ1 domain of its adaptor protein PDZK1.

The PDZ1 domain of the four PDZ domain-containing protein PDZK1 has been reported to bind the C terminus of the HDL receptor scavenger receptor class B, type I (SR-BI), and to control hepatic SR-BI expression and function. We generated wild-type (WT) and mutant murine PDZ1 domains, the mutants bearing single amino acid substitutions in their carboxylate binding loop (Lys(14)-Xaa(4)-Asn(19)-Tyr-Gly-Phe-Phe-Leu(24)), and measured their binding affinity for a 7-residue peptide corresponding to the C terminus of SR-BI ((503)VLQEAKL(509)). The Y20A and G21Y substitutions abrogated all binding activity. Surprisingly, binding affinities (K(d)) of the K14A and F22A mutants were 3.2 and 4.0 µM, respectively, similar to 2.6 µM measured for the WT PDZ1. To understand these findings, we determined the high resolution structure of WT PDZ1 bound to a 5-residue sequence from the C-terminal SR-BI ((505)QEAKL(509)) using x-ray crystallography. In addition, we incorporated the K14A and Y20A substitutions into full-length PDZK1 liver-specific transgenes and expressed them in WT and PDZK1 knock-out mice. In WT mice, the transgenes did not alter endogenous hepatic SR-BI protein expression (intracellular distribution or amount) or lipoprotein metabolism (total plasma cholesterol, lipoprotein size distribution). In PDZK1 knock-out mice, as expected, the K14A mutant behaved like wild-type PDZK1 and completely corrected their hepatic SR-BI and plasma lipoprotein abnormalities. Unexpectedly, the 10-20-fold overexpressed Y20A mutant also substantially, but not completely, corrected these abnormalities. The results suggest that there may be an additional site(s) within PDZK1 that bind(s) SR-BI and mediate(s) productive SR-BI-PDZK1 interaction previously attributed exclusively to the canonical binding of the C-terminal SR-BI to PDZ1.

Normal hepatic cell surface localization of the high density lipoprotein receptor, scavenger receptor class B, type I, depends on all four PDZ domains of PDZK1.

PDZK1 is a four PDZ domain-containing scaffold protein that binds to scavenger receptor class B, type I (SR-BI), the high density lipoprotein receptor, by its first PDZ domain (PDZ1). PDZK1 knock-out mice exhibit a >95% decrease in hepatic SR-BI protein and consequently an approximately 70% increase in plasma cholesterol in abnormally large high density lipoprotein particles. These defects are corrected by hepatic overexpression of full-length PDZK1 but not the PDZ1 domain alone, which partially restores SR-BI protein abundance but not cell surface expression or function. We have generated PDZK1 knock-out mice with hepatic expression of four PDZK1 transgenes encoding proteins with nested C-terminal truncations: pTEM, which lacks the three C-terminal residues (putative PDZ-binding motif), and PDZ1.2, PDZ1.2.3, or PDZ1.2.3.4, which contain only the first two, three, or four N-terminal PDZ domains, respectively, but not the remaining C-terminal sequences. Hepatic overexpression of pTEM restored normal hepatic SR-BI abundance, localization, and function. Hepatic overexpression of PDZ1.2 or PDZ1.2.3 partially restored SR-BI abundance ( approximately 12 or approximately 30% of wild type, respectively) but did not (PDZ1.2) or only slightly (PDZ1.2.3) restored hepatic SR-BI cell surface localization and function. Hepatic overexpression of PDZ1.2.3.4 completely restored SR-BI protein abundance, cell surface expression, and function (normalization of plasma cholesterol levels). Thus, all four PDZ domains in PDZK1, but not PDZ1-3 alone, are sufficient for its normal control of the abundance, localization, and therefore function of hepatic SR-BI, whereas the residues C-terminal to the PDZ4 domain, including the C-terminal putative PDZ-binding domain, are not required.

Overexpression of the PDZ1 domain of PDZK1 blocks the activity of hepatic scavenger receptor, class B, type I by altering its abundance and cellular localization.

PDZK1 is a four-PDZ domain-containing scaffold protein that, via its first PDZ domain (PDZ1), binds to the C terminus of the high density lipoprotein (HDL) receptor scavenger receptor, class B, type I (SR-BI). Abolishing PDZK1 expression in PDZK1 knock-out (KO) mice leads to a post-transcriptional, tissue-specific decrease in SR-BI protein level and an increase in total plasma cholesterol carried in abnormally large HDL particles. Here we show that, although hepatic overexpression of PDZK1 restored normal SR-BI protein abundance and function in PDZK1 KO mice, hepatic overexpression of only the PDZ1 domain was not sufficient to restore normal SR-BI function. In wild-type mice, overexpression of the PDZ1 domain overcame the activity of the endogenous hepatic PDZK1, resulting in a 75% reduction in hepatic SR-BI protein levels and intracellular mislocalization of the remaining SR-BI. As a consequence, the plasma lipoproteins in PDZ1 transgenic mice resembled those in PDZK1 KO mice (hypercholesterolemia due to large HDL). These results indicate that the PDZ1 domain can control the abundance and localization, and therefore the function, of hepatic SR-BI and that structural features of PDZK1 in addition to its SR-BI-binding PDZ1 domain are required for normal hepatic SR-BI regulation.