The presence and subcellular distribution of sterol carrier protein 2 in embryonic-chick tissues

Abstract

Transport of lipids from the yolk to the tissues of the chick embryo is slow during the first 2 weeks of development, but increases abruptly during the last week. Evidence suggests that the lipid traverses the cytoplasm of the yolk-sac membrane before secretion as lipoprotein into the fetal circulation. Little is known about the cytoplasmic transport of lipid in avian systems, but recently the presence of sterol carrier protein 2 (SCP2) was reported in chicken liver. Here we examine the cells of yolk-sac membrane, liver and small intestine for the presence of this protein as a function of the time of embryonic development. The quantity of SCP2 present in the embryonic cells did not appear to correlate with the rate of lipid flux in these tissues. The abrupt appearance of a high-molecular-mass form of SCP2 was detected in small intestine shortly before hatching, but the significance of this protein is not clear. The presence of SCP2 in these tissues was also confirmed by immunocytochemical techniques. Similarly to SCP2 of mammalian cells, avian SCP2 is localized in both peroxisome-like structures and mitochondria. To a lesser extent it is associated with the endoplasmic reticulum.

Sterol Carrier Protein-2, a New Fatty Acyl Coenzyme A-binding Protein

“…This could be accounted for by perhaps the acyl chain (the fluorophore) of the acyl-CoA derivative not inserting quite as far into the acyl chain binding site of SCP-2. The only other study addressing this issue showed that the rotational relaxation time of diphenylhexatriene-PC was 7.4 ns (46). The latter observation is entirely consistent with those obtained herein with parinaric acids and parinaroylCoAs.…”

“…NMR analysis of the SCP-2 at pH 6 showed the polypeptide chain to be arranged in a secondary structure composed of three ␣-helixes and five ␤-sheets, resulting in a slightly elongated globular protein (45). In contrast, an earlier study of apo-SCP-2 time-resolved anisotropy decay at pH 7.4 estimated the rotational relaxation time of apo-SCP-2 at 15 ns and concluded that apo-SCP-2 was a strongly ellipsoidal shaped protein with an axial ratio of 2.8 (46). Neither the NMR study (45) nor the present data support this conclusion.…”

“…Neither the NMR study (45) nor the present data support this conclusion. We suggest that the reason for the apparent discrepancy may be due to the presence of SCP-2 dimers in the earlier study wherein 10 M apo-SCP-2 at pH 7.4 was examined (46). At 10 M apo-SCP-2, but not at 0.2-8.0 M apo-SCP-2 (data shown herein), apo-SCP-2 can form dimers with a rotational relaxation time of 15.7 ns (data not shown).…”

“…The latter observation is entirely consistent with those obtained herein with parinaric acids and parinaroylCoAs. Furthermore, it also suggests that binding of the diphenylhexatriene-PC ligand (rotational relaxation time of 7.4 ns) to dimeric apo-SCP-2 (rotational relaxation time near 15 ns) may disrupt SCP-2 dimers (46).…”

Lipid specificity and location of the sterol carrier protein‐2 fatty acid‐binding site: A fluorescence displacement and energy transfer study

“…. While some of these, such as fatty-acid binding proteins (FABP) and fatty acyl CoA binding protein, appear localized primarily to the cell cytoplasm (2,3), others such as sterol carrier protein-2 not only are found in cytoplasm but also are highly enriched in peroxisomes (4)(5)(6)(7). The origin and function of the cytoplasmic form of SCP-2 are unclear at this time.…”