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Role of the hydrophobic domain in targeting caveolin-1 to lipid droplets.

Ostermeyer AG, Ramcharan LT, Zeng Y, Lublin DM, Brown DA - J. Cell Biol. (2004)

Bottom Line: Next, we found that a mutant H-Ras, present on the cytoplasmic surface of the ER but lacking a hydrophobic peptide domain, did not accumulate on LDs.The hydrophobic domain, but no specific sequence therein, was required for LD targeting of caveolin-1.We propose that proper packing of putative hydrophobic helices may be required for LD targeting of caveolin-1.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry and Cell Biology, State University of New York at Stony Brook, Stony Brook, NY 11794-5215, USA.

ABSTRACT
Although caveolins normally reside in caveolae, they can accumulate on the surface of cytoplasmic lipid droplets (LDs). Here, we first provided support for our model that overaccumulation of caveolins in the endoplasmic reticulum (ER) diverts the proteins to nascent LDs budding from the ER. Next, we found that a mutant H-Ras, present on the cytoplasmic surface of the ER but lacking a hydrophobic peptide domain, did not accumulate on LDs. We used the fact that wild-type caveolin-1 accumulates in LDs after brefeldin A treatment or when linked to an ER retrieval motif to search for mutants defective in LD targeting. The hydrophobic domain, but no specific sequence therein, was required for LD targeting of caveolin-1. Certain Leu insertions blocked LD targeting, independently of hydrophobic domain length, but dependent on their position in the domain. We propose that proper packing of putative hydrophobic helices may be required for LD targeting of caveolin-1.

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Model depicting role of hydrophobic helix packing in LD targeting of caveolin-1. We speculate here that the hydrophobic domain of caveolin-1 forms two α helices separated by a tight turn. (A) We propose that correct packing of the putative hydrophobic helices is required for LD targeting of wild-type caveolin-1. (B) Insertion of bulky Leu residues in both helices could inhibit LD targeting sterically by preventing proper helix packing. (C) Helical wheel depiction of the first half of the hydrophobic domain of caveolin-1 (R101 at the membrane interface [underlined] through Y119 [*]). Circles, residues on a Gly + Ala–rich helix face. (D) L144 (underlined) through A165 of the core coding region of HCV strain Glasgow (genotype 1a; Hope and McLauchlan, 2000), just downstream of the P138 + P143 motif, modeled as an α helix. Circles, residues on a Gly + Ala–rich helix face. Squares, charged residues.
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fig9: Model depicting role of hydrophobic helix packing in LD targeting of caveolin-1. We speculate here that the hydrophobic domain of caveolin-1 forms two α helices separated by a tight turn. (A) We propose that correct packing of the putative hydrophobic helices is required for LD targeting of wild-type caveolin-1. (B) Insertion of bulky Leu residues in both helices could inhibit LD targeting sterically by preventing proper helix packing. (C) Helical wheel depiction of the first half of the hydrophobic domain of caveolin-1 (R101 at the membrane interface [underlined] through Y119 [*]). Circles, residues on a Gly + Ala–rich helix face. (D) L144 (underlined) through A165 of the core coding region of HCV strain Glasgow (genotype 1a; Hope and McLauchlan, 2000), just downstream of the P138 + P143 motif, modeled as an α helix. Circles, residues on a Gly + Ala–rich helix face. Squares, charged residues.

Mentions: As schematized in Fig. 9 (A and B), we suggest that proper packing of the two putative hydrophobic helices is important in LD targeting of caveolin-1. Seven Leu would form two turns of an α helix. We speculate that the bulky Leu side chains interfere with correct packing of the helices with each other, or with other membrane proteins. One 7-Leu insertion alone or insertion of 14 Leu near one end of the hydrophobic domain might be tolerated. However, simultaneous insertion of seven Leu into both helices might disrupt helix packing enough to block LD targeting, particularly if the helices normally pack against each other. Consistent with this possibility, as shown in Fig. 9 C, one face of a helix formed by the first half of the hydrophobic domain would consist entirely of Gly and Ala, whose small side chains are highly favored on hydrophobic helix faces that pack closely with other helices in membranes (Eilers et al., 2002).


Role of the hydrophobic domain in targeting caveolin-1 to lipid droplets.

Ostermeyer AG, Ramcharan LT, Zeng Y, Lublin DM, Brown DA - J. Cell Biol. (2004)

Model depicting role of hydrophobic helix packing in LD targeting of caveolin-1. We speculate here that the hydrophobic domain of caveolin-1 forms two α helices separated by a tight turn. (A) We propose that correct packing of the putative hydrophobic helices is required for LD targeting of wild-type caveolin-1. (B) Insertion of bulky Leu residues in both helices could inhibit LD targeting sterically by preventing proper helix packing. (C) Helical wheel depiction of the first half of the hydrophobic domain of caveolin-1 (R101 at the membrane interface [underlined] through Y119 [*]). Circles, residues on a Gly + Ala–rich helix face. (D) L144 (underlined) through A165 of the core coding region of HCV strain Glasgow (genotype 1a; Hope and McLauchlan, 2000), just downstream of the P138 + P143 motif, modeled as an α helix. Circles, residues on a Gly + Ala–rich helix face. Squares, charged residues.
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Related In: Results  -  Collection

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fig9: Model depicting role of hydrophobic helix packing in LD targeting of caveolin-1. We speculate here that the hydrophobic domain of caveolin-1 forms two α helices separated by a tight turn. (A) We propose that correct packing of the putative hydrophobic helices is required for LD targeting of wild-type caveolin-1. (B) Insertion of bulky Leu residues in both helices could inhibit LD targeting sterically by preventing proper helix packing. (C) Helical wheel depiction of the first half of the hydrophobic domain of caveolin-1 (R101 at the membrane interface [underlined] through Y119 [*]). Circles, residues on a Gly + Ala–rich helix face. (D) L144 (underlined) through A165 of the core coding region of HCV strain Glasgow (genotype 1a; Hope and McLauchlan, 2000), just downstream of the P138 + P143 motif, modeled as an α helix. Circles, residues on a Gly + Ala–rich helix face. Squares, charged residues.
Mentions: As schematized in Fig. 9 (A and B), we suggest that proper packing of the two putative hydrophobic helices is important in LD targeting of caveolin-1. Seven Leu would form two turns of an α helix. We speculate that the bulky Leu side chains interfere with correct packing of the helices with each other, or with other membrane proteins. One 7-Leu insertion alone or insertion of 14 Leu near one end of the hydrophobic domain might be tolerated. However, simultaneous insertion of seven Leu into both helices might disrupt helix packing enough to block LD targeting, particularly if the helices normally pack against each other. Consistent with this possibility, as shown in Fig. 9 C, one face of a helix formed by the first half of the hydrophobic domain would consist entirely of Gly and Ala, whose small side chains are highly favored on hydrophobic helix faces that pack closely with other helices in membranes (Eilers et al., 2002).

Bottom Line: Next, we found that a mutant H-Ras, present on the cytoplasmic surface of the ER but lacking a hydrophobic peptide domain, did not accumulate on LDs.The hydrophobic domain, but no specific sequence therein, was required for LD targeting of caveolin-1.We propose that proper packing of putative hydrophobic helices may be required for LD targeting of caveolin-1.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry and Cell Biology, State University of New York at Stony Brook, Stony Brook, NY 11794-5215, USA.

ABSTRACT
Although caveolins normally reside in caveolae, they can accumulate on the surface of cytoplasmic lipid droplets (LDs). Here, we first provided support for our model that overaccumulation of caveolins in the endoplasmic reticulum (ER) diverts the proteins to nascent LDs budding from the ER. Next, we found that a mutant H-Ras, present on the cytoplasmic surface of the ER but lacking a hydrophobic peptide domain, did not accumulate on LDs. We used the fact that wild-type caveolin-1 accumulates in LDs after brefeldin A treatment or when linked to an ER retrieval motif to search for mutants defective in LD targeting. The hydrophobic domain, but no specific sequence therein, was required for LD targeting of caveolin-1. Certain Leu insertions blocked LD targeting, independently of hydrophobic domain length, but dependent on their position in the domain. We propose that proper packing of putative hydrophobic helices may be required for LD targeting of caveolin-1.

Show MeSH
Related in: MedlinePlus