Abstract
Acidification in the endosome causes lipoprotein release by promoting a conformational change in the LDLR allowing its recycling and degradation of LDL. Notwithstanding conformational changes occurring in the LDLR have expanded considerably, structural changes occurring in LDL particles have not been fully explored yet. The objectives of the present work were to study structural changes occurring in apoB100 by infrared spectroscopy (IR) and also LDL size and morphology by dynamic light scattering (DLS) and electron microscopy (EM) at both pH 7.4 and 5.0. We determined by IR that pH acidification from 7.4 to 5.0, resembling that occurring within endosomal environment, induces a huge reversible structural rearrangement of apoB100 that is characterized by a reduction of beta-sheet content in favor of alpha-helix structures. Data obtained from DLS and EM showed no appreciable differences in size and morphology of LDL. These structural changes observed in apoB100, which are likely implied in particle release from lipoprotein receptor, also compromise the apoprotein stability what would facilitate LDL degradation. In conclusion, the obtained results reveal a more dynamic picture of the LDL/LDLR dissociation process than previously perceived and provide new structural insights into LDL/LDLR interactions than can occur at endosomal low-pH milieu.
Highlights
LDL receptor (LDLR) is responsible for the uptake of LDL particles into cells[1] upon binding to apoB100, the integral structural apolipoprotein for LDL particles[2]
Some regions of the apoB100 rich in β-type structures are embebded in the phospholipid monolayer of the particle[2,12,13], while the residues involved in LDLR binding are exposed to the medium[12]
LDL morphology was examined by NS-electron microscopy (EM) at pH 7.4 and pH 5.0, all the particles examined in the images were approximately circular, consistent with a spherical shape (Fig. 1B,C)
Summary
LDL receptor (LDLR) is responsible for the uptake of LDL particles into cells[1] upon binding to apoB100, the integral structural apolipoprotein for LDL particles[2]. Knowledge on LDLR conformational changes occurring in the cargo releasing process has expanded considerably, structural changes occurring in LDL particles have not been explored yet. Acidic pH may alter the conformation of apoB100, the characteristics of the LDL particle or both, contributing to the cargo releasing process. The main objective of the present study was to evaluate the structural changes in apoB100 and lipoprotein particle occurring upon pH acidification, mimicking the acidic environment of the endosome. The results show differences in the apoB100 secondary structure content among lipoproteins incubated at pH 7.4 or at pH 5.0 that could resemble those occurring in the early or late endosomes, respectively. To our knowledge this work presents a new approach for understanding the LDLR/LDL releasing process occurring upon endosomal acidification
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