Identification of a novel N-terminal hydrophobic sequence that targets proteins to lipid droplets.
AAM-B is a putative methyltransferase that is a resident protein of lipid droplets. We have identified an N-terminal 28 amino acid hydrophobic sequence that is necessary and sufficient for targeting the protein to droplets. This sequence will also insert AAM-B into the endoplasmic reticulum (ER). A similar hydrophobic sequence (1-23) in the cytochrome p450 2C9 cannot substitute for 1-28 and only inserts AAM-B into the ER, which indicates that hydrophobicity and ER anchoring are not sufficient to reach the droplet. We found that a similar N-terminal hydrophobic sequence in cytochrome b5 reductase 3 and ALDI could also heterologously target proteins to droplets. Targeting is not affected by changing a conserved proline residue that potentially facilitates the formation of a hairpin loop to leucine. By contrast, targeting is blocked when AAM-B amino acids 59-64 or 65-70, situated downstream of the hydrophobic sequence, are changed to alanines. AAM-B-GFP expressed in Saccharomyces cerevisiae is also faithfully targeted to lipid bodies, indicating that the targeting mechanism is evolutionarily conserved. In conclusion, a class of hydrophobic sequences exists that when placed at the N-terminus of a protein will cause it to accumulate in droplets and in the ER.
Zehmer
John K
JK
Department of Cell Biology, University of Texas Southwestern Medical Center, Dallas, TX 75390-9039, USA.
Bartz
René
R
Liu
Pingsheng
P
Anderson
Richard G W
RG
D000595:Amino Acid Sequence / Q000502:physiology; D019943:Amino Acid Substitution / Q000502:physiology; D000818:Animals; D019556:COS Cells; D002522:Chlorocebus aethiops; D003593:Cytoplasm / Q000378:metabolism* / Q000648:ultrastructure; D004721:Endoplasmic Reticulum / Q000378:metabolism* / Q000648:ultrastructure; D006367:HeLa Cells; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D002479:Inclusion Bodies / Q000378:metabolism* / Q000648:ultrastructure; D007425:Intracellular Membranes / Q000378:metabolism / Q000648:ultrastructure; D050356:Lipid Metabolism / Q000502:physiology; D008055:Lipids / Q000502:physiology*; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism*; D008780:Methyltransferases / Q000737:chemistry* / Q000378:metabolism*; D044367:Phase Transition; D017434:Protein Structure, Tertiary / Q000502:physiology; D021381:Protein Transport / Q000502:physiology; D012441:Saccharomyces cerevisiae / Q000378:metabolism
D008055:Lipids; D008565:Membrane Proteins; C000607000:AAM-B protein, human; D008780:Methyltransferases