Abstract
Sterol carrier protein-2 (SCP2), also called nonspecific lipid-transfer protein, is thought to play a major role in intracellular lipid transport and metabolism, and it has been associated with diseases involving abnormalities in lipid trafficking, such as Zellweger syndrome. The Scp2 gene encodes the 58 kDa sterol carrier protein-x (SCPX) and 15 kDa pro-SCP2 proteins, both of which contain a 13 kDa SCP2 domain in their C-termini. We found that 22-NBD-cholesterol, a fluorescent analog of cholesterol and a preferred SCP2 ligands, was not localized in the peroxisomes. This raises questions about previous reports on the localization of the SCPX and SCP2 proteins and their relationship to peroxisomes and mitochondria in intracellular cholesterol transport. Immunofluorescent staining of cryosections of mouse testis and of MA-10 mouse tumor Leydig cells showed that SCPX and SCP2 are present in both mouse testicular interstitial tissue and in MA-10 cells. Fluorescent fusion proteins of SCPX and SCP2, as well as confocal live-cell imaging, were used to investigate the subcellular targeting of these proteins and the function of the putative mitochondrial targeting sequence. The results showed that SCPX and SCP2 are targeted to the peroxisomes by the C-terminal PTS1 domain, but the putative N-terminal mitochondrial targeting sequence alone is not potent enough to localize SCPX and SCP2 to the mitochondria. Homology modeling and molecular docking studies indicated that the SCP2 domain binds cholesterol, but lacks specificity of the binding and/or transport. These findings further our understanding of the role of SCPX and SCP2 in intracellular cholesterol transport, and present a new point of view on the role of these proteins in cholesterol trafficking.
Highlights
Sterol carrier protein-2 (SCP2), known as nonspecific lipid transfer protein, is a lipidbinding protein associated with many proposed functions in lipid metabolism
Based on the preliminary findings that 22-NBD-cholesterol did not localize to the peroxisomes in the MA-10 cell line (Fig 1), and given that both sterol carrier protein-x (SCPX) and SCP2 are associated with peroxisomes via their PTS1 sequences whereas SCP2 is closely related to mitochondria (Fig 5), further investigations were conducted to elucidate the subcellular localization of the SCPX and SCP2 proteins and, subsequently, the role of these proteins in the intracellular localization of 22-NBD-cholesterol
From our preliminary RNA sequencing (RNA-seq) data performed on the whole transcriptome of the cells [27], we found that transcripts for both Scpx and Scp2 demonstrate low expression in MA-10 cells
Summary
Sterol carrier protein-2 (SCP2), known as nonspecific lipid transfer protein, is a lipidbinding protein associated with many proposed functions in lipid metabolism. Sterol Carrier Protein-2 in Intracellular Cholesterol Trafficking contains two transcription initiation sites, giving rise to the 58 kDa sterol carrier protein-x (SCPX) and 15 kDa pro-SCP2 proteins, both of which contain the complete 13 kDa SCP2 protein or domain sequence in their C-termini [1,2]. SCP2 is likely involved in lipid trafficking and metabolism in general, as it has been found to bind to and participate in the metabolism of fatty acids [5,6,7], fatty acyl-CoAs [8,9], cholesteryl esters [10], phospholipids [11,12,13], and various other lipids. SCP2 is thought to be a soluble sterol carrier because of its ability to bind to sterols and its localization to multiple organelles, such as the peroxisomes, mitochondria, endoplasmic reticulum, and cytosol [14,15,16]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
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 call
