HAU16008-03-5 - GG E T T NN GAA C R O S STT H EM E E M B R...

Info icon This preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
R E V I E W Protein Translocation Across Biological Membranes William Wickner 1 * and Randy Schekman 2 * Subcellular compartments have unique protein compositions, yet protein synthesis only occurs in the cytosol and in mitochondria and chloroplasts. How do proteins get where they need to go? The first steps are targeting to an organelle and efficient translocation across its limiting membrane. Given that most transport systems are exquisitely substrate specific, how are diverse protein sequences recognized for translocation? Are they translocated as linear polypeptide chains or after folding? During translocation, how are diverse amino acyl side chains accommodated? What are the proteins and the lipid environment that catalyze transport and couple it to energy? How is translocation coordinated with protein synthesis and folding, and how are partially translocated transmembrane proteins released into the lipid bilayer? We review here the marked progress of the past 35 years and salient questions for future work. Emerging technologies have enabled progress in understanding protein translocation. In vitro protein synthesis led to the discovery that the mRNAs for secreted proteins are attached to membranes, whereas cytosolic proteins are made on free polysomes ( 1–3 ). The junction between endoplasmic reticulum (ER)–bound polysomes and the membrane is tight enough to exclude protease ( 4 ), and nascent chains are discharged directly into the lumen ( 5 ). Mouse myeloma mRNA was found to encode a poly- peptide 1.5 kD larger than mature immuno- globulin light chain ( 6 ). The larger form was postulated to be a precursor with an N-terminal extension that specifies secretion, a research direction that culminated in the signal hypoth- esis ( 7 ). B Signal sequences [ were deciphered for proteins that crossed a membrane, includ- ing the ER, mitochondria, chloroplast, and bacterial envelope. Armed with a signal sequence, mature domains merely had to be susceptible to unfolding in order to translocate ( 8 , 9 ). The signal sequences of each organelle have shared motifs of polarity and structure but no sequence conservation. For example, bac- terial or ER export signals are basic at the N terminus, followed by a stretch of 8 to 14 apolar residues and a short cleavage motif that is recognized by a dedicated peptidase. Bacte- rial sec (secretion) genes, encoding proteins that support translocation, were identified either as suppressors of signal sequence mutants ( 10 ) or as temperature-sensitive mutants that failed to initiate secretion at the nonpermissive temperature ( 11 ). The yeast Sec61p ( 12 ) is homologous to bacterial SecY, establishing that the membrane-embedded portion of these translocons is conserved. In vitro translocation reactions were developed by adding isolated organelles to protein synthe- sis extracts for ER ( 7 ), mitochondria ( 13 , 14 ), chloroplasts ( 15 ), and bacterial plasma mem- brane ( 16 , 17 ). These in vitro systems were the basis for discovering energy requirements, determining whether translocation needed chaperones or ongoing protein synthesis, and
Image of page 1

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern