Identification of a novel N-terminal hydrophobic sequence that targets proteins to lipid droplets

• Published in: Journal of cell science Vol. 121; no. 11; pp. 1852 - 1860
• Main Authors: Zehmer, John K, Bartz, René, Liu, Pingsheng, Anderson, Richard G.W
• Format: Journal Article
• Language: English
• Published: England The Company of Biologists Limited 01-06-2008
• Subjects: Amino Acid Sequence - physiology; Amino Acid Substitution - physiology; Animals; Cercopithecus aethiops; COS Cells; Cytoplasm - metabolism; Cytoplasm - ultrastructure; Endoplasmic Reticulum - metabolism; Endoplasmic Reticulum - ultrastructure; HeLa Cells; Humans; Hydrophobic and Hydrophilic Interactions; Inclusion Bodies - metabolism; Inclusion Bodies - ultrastructure; Intracellular Membranes - metabolism; Intracellular Membranes - ultrastructure; Lipid Metabolism - physiology; Lipids - physiology; Membrane Proteins - chemistry; Membrane Proteins - metabolism; Methyltransferases - chemistry; Methyltransferases - metabolism; Phase Transition; Protein Structure, Tertiary - physiology; Protein Transport - physiology; Saccharomyces cerevisiae; Saccharomyces cerevisiae - metabolism
• ISSN: 0021-9533; 1477-9137
• DOI: 10.1242/jcs.012013

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.