Signal sequence-dependent function of the TRAM protein during early phases of protein transport across the endoplasmic reticulum membrane.
Cotranslational translocation of proteins across the mammalian ER membrane involves, in addition to the signal recognition particle receptor and the Sec61p complex, the translocating chain-associating membrane (TRAM) protein, the function of which is still poorly understood. Using reconstituted proteoliposomes, we show here that the translocation of most, but not all, secretory proteins requires the function of TRAM. Experiments with hybrid proteins demonstrate that the structure of the signal sequence determines whether or not TRAM is needed. Features that distinguish TRAM-dependent and -independent signal sequences include the length of their charged, NH2-terminal region and the structure of their hydrophobic core. In cases where TRAM is required for translocation, it is not needed for the initial interaction of the ribosome/nascent chain complex with the ER membrane but for a subsequent step inside the membrane in which the nascent chain is inserted into the translocation site in a protease-resistant manner. Thus, TRAM functions in a signal sequence-dependent manner at a critical, early phase of the translocation process.
Voigt
S
S
Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115, USA.
Jungnickel
B
B
Hartmann
E
E
Rapoport
T A
TA
D000595:Amino Acid Sequence; D000818:Animals; D001692:Biological Transport; D002474:Cell-Free System; D004285:Dogs; D018870:Endoplasmic Reticulum, Rough / Q000378:metabolism*; D008562:Membrane Glycoproteins / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism; D008969:Molecular Sequence Data; D021382:Protein Sorting Signals / Q000502:physiology*; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism; D018000:Receptors, Peptide / Q000378:metabolism; D000069816:SEC Translocation Channels; D012995:Solubility; D013329:Structure-Activity Relationship
D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D021382:Protein Sorting Signals; D018160:Receptors, Cytoplasmic and Nuclear; D018000:Receptors, Peptide; D000069816:SEC Translocation Channels; C036318:signal peptide receptor; C074282:translocating chain-associating membrane protein (TRAM)