High Density Lipoprotein Endocytosis by Scavenger Receptor SR-BII Is Clathrin-dependent and Requires a Carboxyl-terminal Dileucine Motif

Abstract

The high density lipoprotein (HDL) receptor Scavenger Receptor BII (SR-BII) is encoded by an alternatively spliced mRNA from the SR-BI gene and is expressed in various tissues. SR-BII protein differs from SR-BI only in the carboxyl-terminal cytoplasmic tail, which, as we showed previously, must contain a signal that confers predominant intracellular expression and rapid endocytosis of HDL. We haveshownthatSR-BIImediatesHDLendocytosisthroughaclathrindependent, caveolae-independent pathway. Two candidate amino acid motifs were identified in the tail that could mediate association with clathrin-containing endocytic vesicles: a putative dileucine motif at position 492-493 and an overlapping tyrosine-based YXXZ motif starting at position 489. Although substitution of tyrosine at position 489 with alanine or histidine did not affect endocytosis, substitution L492A resulted in increased surface binding of HDL and reduced HDL particle endocytosis. Substitution L493A had a less dramatic effect. No other regions in the carboxyl-terminal tail appeared to contain motifs required for HDL endocytosis. Substitutions of leucine at position 492 with the hydrophobic amino acids valine or phenylalanine also reduced HDL endocytosis, stressing the importance of leucine at this position. Introducing the SR-BII YTPLL motif into the carboxyl-terminal cytoplasmic tail of SR-BI converted SR-BI into an endocytic receptor resembling SR-BII. These results demonstrated that SR-BII differs from SR-BI in subcellular localization and trafficking and suggest that the two isoforms differ in the manner in which they target ligands intracellularly.

Regulation of SR-BI-mediated selective lipid uptake in Chinese hamster ovary-derived cells by protein kinase signaling pathways

“…Nevertheless, our data suggest that mSR-BI endocytosis versus retention on the cell surface may be a step at which SR-BI's selective uptake activity can be regulated. This is consistent with recent studies that suggest that the lower selective uptake activity of SR-BII, a splice variant of SR-BI, which has a different C-terminal cytoplasmic tail, appears to be the result of increased endocytosis and that the introduction of an endocytic motif into the carboxy-terminal cytoplasmic tail of SR-BI led to its increased endocytosis and decreased selective lipid uptake activity (20).…”

“…Similarly, SR-BI-mediated sterol uptake by hepatocytes and trafficking to the bile canalicular (apical) space does not appear to be energy dependent (17,18). Furthermore, the low selective uptake activities of SR-BII, a splice variant of SR-BI with a different carboxyterminal cytoplasmic tail, and of a mutant form of SR-BI with an endocytic sequence inserted into its carboxyterminal tail appear to be due to their increased endocytosis (19)(20)(21). On the other hand, mSR-BI does mediate the endocytosis and intracellular accumulation of a variety of other ligands, including serum amyloid a, lipopolysaccharide, and apoptotic cells (22)(23)(24)(25)(26).…”

HSL-knockout mouse testis exhibits class B scavenger receptor upregulation and disrupted lipid raft microdomains

“…In the present study, in the HSL +/+ control mouse testis, we show that SR-BI is located in the membrane of Leydig cells and on elongated spermatids, which agrees with previous studies in other species ( 57,59,60 ), including humans ( 35 of the SR-BI mature form (82 kDa) and caveolin-1 in lowdensity membrane fractions does not necessarily indicate that both proteins are within the same morphological structure in the cell. The possibility exists that SR-BI localizes to lipid rafts not containing caveolin-1.…”

Phosphatidylinositol-3-Kinase Regulates Scavenger Receptor Class B Type I Subcellular Localization and Selective Lipid Uptake in Hepatocytes

“…9,23 Plasmids encoding EGFP-SRBI or EGFP alone were electroporated into cells using a nucleofector (Amaxa Biosystems). Cells were used 24 to 48 hours after transfection.…”