Stx5 is a novel interactor of VLDL-R to affect its intracellular trafficking and processing

Abstract

We identified syntaxin 5 (Stx5), a protein involved in intracellular vesicle trafficking, as a novel interaction partner of the very low density lipoprotein (VLDL)-receptor (VLDL-R), a member of the LDL-receptor family. In addition, we investigated the effect of Stx5 on VLDL-R maturation, trafficking and processing. Here, we demonstrated mutual association of both proteins using several in vitro approaches. Furthermore, we detected a special maturation phenotype of VLDL-R resulting from Stx5 overexpression. We found that Stx5 prevented advanced Golgi-maturation of VLDL-R, but did not cause accumulation of the immature protein in ER, ER to Golgi compartments, or cis-Golgi ribbon, the main expression sites of Stx5. Rather more, abundantly present Stx5 was capable of translocating ER-/N-glycosylated VLDL-R to the plasma membrane, and thus was insensitive to BFA treatment and low temperature. Furthermore, abundant presence of Stx5 significantly interfered with VLDL-R reaching the trans-Golgi network. Based on our findings, we postulate that Stx5 can directly bind to the C-terminal domain of VLDL-R, thereby influencing the receptor's glycosylation, trafficking and processing characteristics. Resulting from that, we further suggest that Stx5 might play a role in modulating VLDL-R physiology by participating in an abrasively described or completely novel Golgi-bypass pathway.

Keywords: APP; ApoE; ApoER2; BFA; C-terminal fragment; C-terminal fragment of ubiquitin; COP; CTF; Cub; Dab1; EGF; ER; ER-Golgi intermediate compartment/vesicular tubular carriers; ERGIC/VTC; GST; Glycosylation; HEK; Intracellular trafficking; KLH; LDL-R; LRP; Maturation; N-ethylmaleimide-sensitive factor-attachment protein receptor; N-terminal fragment of ubiquitin; Nub; PS; PTB; RT; SNARE; SNX17; Stx; Stx5; VLD; VLDL-R; aRH; aa; amino acid; amyloid precursor protein; apolipoprotein E; apolipoprotein E receptor 2; autosomal recessive hypercholesterolaemia; brefeldin A; coatamer protein; disabled 1; endoplasmic reticulum; endoribonuclease-prepared siRNA; epidermal growth factor; esiRNA; glutathione S-transferase; human embryonic kidney; kDa; keyhole limpet haemocyanin; kilo Dalton; low density lipoprotein receptor-related protein; low density lipoprotein-receptor; phosphotyrosine binding; presenilin; room temperature; sortin nexin 17; syntaxin; very low density lipoprotein; very low density lipoprotein-receptor.