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

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 have shown that SR-BII mediates HDL endocytosis through aclathrin-dependent, 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.