Abstract
Studies in different liver-derived cells in culture indicate that apolipoprotein (apo) B-100 production is regulated largely by intracellular degradation and the ubiquitin-proteasome pathway is a major mechanism for the degradation. The proteasomal degradation of apoB-100 was postulated to be an intrinsic property of the protein that occurs even in the presence of optimal amounts of lipids supplied to the cell. We examined apoB-100 and apoB-48 biogenesis in CaCo2, a human colon carcinoma cell line. To our surprise, apoB-100 and apoB-48 were quantitatively secreted by CaCo2 cells; essentially none of the newly synthesized apoB was degraded before secretion in a 2-h period whether the cells were cultured on filter or on plastic. Furthermore, although ubiquitin immunoreactivity was readily detected in the intracellular apoB isolated from HepG2 cells, little or no ubiquitin was detectable in the intracellular apoB from CaCo2 cells. The amounts of free ubiquitin and total and non-apoB ubiquitinated proteins were comparable in HepG2 and CaCo2 cells, indicating that CaCo2 cells have the necessary machinery for tagging ubiquitin chains onto cellular proteins for proteasomal degradation. Incubation in lipoprotein-deficient serum did not induce apoB degradation, but the addition of a microsomal triglyceride transfer protein inhibitor led to apoB degradation in CaCo2 cells. Finally, similar proportions of apoB polypeptide in isolated microsomes from CaCo2 and HepG2 cells were accessible to exogenously added trypsin, indicating that the mere exposure of apoB nascent chains to the cytosolic compartment is insufficient to cause the proteasomal degradation. Therefore, the intracellular degradation of apoB is not an intrinsic property of the protein, and the phenomenon is neither universal nor inevitable. The unconditional use of apoB as a paradigm for intracellular protein degradation is not warranted.
Highlights
Voltage-dependent potassium channels of the Shaker or Kv1 family play a fundamental role in the mammalian nervous system by determining resting membrane potential, frequency of action potential firing, and neurotransmitter release [2]
Previous studies have shown that coexpression with either Kv1.1 or Kv2 increased 125I-dendrotoxin binding in cells expressing Kv1.2 [15] and that the autonomous highly conserved core domain is sufficient to mediate increases in current amplitude of coexpressed Kv1.4 ␣ subunits [21]
We addressed whether each wild-type Kv subunit harboring this core domain would display the effects on Kv1.2 intracellular trafficking and surface expression observed previously for Kv2 [15]
Summary
Voltage-dependent potassium channels of the Shaker or Kv1 family play a fundamental role in the mammalian nervous system by determining resting membrane potential, frequency of action potential firing, and neurotransmitter release [2]. These data suggest that NADP؉ binding and/or the integrity of the binding pocket structure, but not catalytic activity, of Kv subunits is required for intracellular trafficking of Kv1 channel complexes in mammalian cells and for axonal targeting in neurons.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
