Transport of proteins across membranes — a paradigm in transition

Isenman, Lois; Liebow, Charles; Rothman, Stephen

doi:10.1016/0304-4157(95)00009-7

Cited by 22 publications

(17 citation statements)

References 155 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…In this paper we provide for the first time direct experimental evidence that OEP75 constitutes a voltage‐gated transmembrane channel with a hydrophilic pore. The results presented in this paper add further experimental evidence and support current concepts on the mechanism of protein translocation across the membrane (Isenman et al ., 1995; Schnell, 1995; Soll, 1995; Schatz and Dobberstein, 1996).…”

Section: Discussionsupporting

confidence: 89%

Reconstitution of a chloroplast protein import channel

et al. 1997

View full text Add to dashboard Cite

The chloroplastic outer envelope protein OEP75 with a molecular weight of 75 kDa probably forms the central pore of the protein import machinery of the outer chloroplastic membrane. Patch-clamp analysis shows that heterologously expressed, purified and reconstituted OEP75 constitutes a voltage-gated ion channel with a unit conductance of Λ ϭ 145pS. Activation of the OEP75 channel in vitro is completely dependent on the magnitude and direction of the voltage gradient. Therefore, movements of protein charges of parts of OEP75 in the membrane electric field are required either for pore formation or its opening. In the presence of precursor protein from only one side of the bilayer, strong flickering and partial closing of the channel was observed, indicating a specific interaction of the precursor with OEP75. The comparatively low ionic conductance of OEP75 is compatible with a rather narrow aqueous pore (d pore ≅ 8-9 Å). Provided that protein and ion translocation occur through the same pore, this implies that the environment of the polypeptide during the transit is mainly hydrophilic and that protein translocation requires almost complete unfolding of the precursor.

show abstract

Section: Discussionsupporting

confidence: 89%

Reconstitution of a chloroplast protein import channel

et al. 1997

View full text Add to dashboard Cite

show abstract

“…However, it might be expected that some ion leakage through protein translocases could be tolerated, simply by increasing expenditure of energy to maintain ion gradients across membranes (33). It is not obvious how a translocation machinery could nonselectively allow transport of a variety of substrates while selectively excluding small ions.…”

Section: Discussionmentioning

confidence: 99%

Energy-transducing thylakoid membranes remain highly impermeable to ions during protein translocation

Teter

Theg

1998

Proc. Natl. Acad. Sci. U.S.A.

View full text Add to dashboard Cite

We investigated the operation of a posttranslational protein translocation pathway to determine whether ions are excluded from the translocase during protein transport. The membrane capacitance during protein translocation across chloroplast thylakoid membranes was monitored via electric-field-indicating carotenoid electrochromic bandshift measurements. Evidence is presented that shows that the membrane ion conductance is not increased during the complete cycle of binding, transport, and substrate release by the ⌬pH-dependent translocase; i.e., the membrane remains iontight during protein translocation. We further demonstrate that a synthetic targeting peptide that directs proteins across this membrane does not gate translocation pores. We conclude that protein transport across the thylakoid membrane does not compromise its ability to maintain ion gradients and is, thus, unlikely to affect its functions in energy transduction.Although a number of experiments indicate that many proteins traverse membranes through aqueous pores, passing from N terminus to C terminus in an extended conformation (1), it is becoming increasingly apparent that some translocases are capable of transporting proteins possessing a variety of structures. Mitochondria and bacterial membrane transporters can accommodate branched (2) and disulfide-bridged (3) proteins, respectively. Chloroplasts have recently been shown to import fully folded polypeptides (4, 5) and the import of oligomerized proteins and even gold particles has been demonstrated in peroxisomes (6-8). In addition, it has been shown for several translocases that the same machinery mediates the transport of both soluble and integral membrane proteins (9, 10). These observations indicate that protein translocases are dynamic and versatile in their ability to transport substrates with different structures into diverse environments. Because proteins have irregular surfaces and translocators have broad specificities, the question arises as to whether ion leakage accompanies protein transport through a pore, thereby compromising the ion permeability barrier of the membrane. This issue has been examined by using artificially induced membrane-spanning translocation intermediates, with some researchers describing an attendant ion leakage and others reporting no accompanying change in ion permeability (11)(12)(13). In addition, the presumed interactions of different targeting peptides with translocators have been reported to alter ion flux, in some cases by opening ion-conducting channels and in other cases by transiently blocking ion movement through active channels (14 -18). In one such report, substratedependent proton flux through the bacterial protein secretion machinery was observed, albeit only when some of the translocon components were overproduced (19). In the present study, we used a native precursor to examine the total ion conductance of the energy-transducing chloroplast thylakoid membrane during uninterrupted translocation cycles of a protein transporter. MATERIAL...

show abstract

“…It is possible that the constitutive, maturative, or another as yet unknown pathway that operates in addition to exocytosis is responsible for the differential secretion of lingual lipase and amylase. In a recent review article (25), it was suggested that processes, in addition to exocytosis, for the secretion of digestive enzymes would be important to allow for modulation of the amounts of different enzymes secreted depending upon the content of the food ingested. Whereas, exocytosis allows for the secretion of stored enzymes that have been previously synthesized, other means of secretion could modulate the synthesis and independent, immediate secretion of enzymes needed to digest a meal that required more of a particular enzyme.…”

Section: Resultsmentioning

confidence: 99%

Effect of Substance P and Receptor Antagonists on Secretion of Lingual Lipase and Amylase from Rat von Ebner's Gland

Field

Chirtel

Redman³

1997

Peptides

View full text Add to dashboard Cite

Substance P (SP, 1 microM) when incubated with minced von Ebner's glands for 15, 30, and 60 min, stimulated secretion of lingual lipase (12.14% +/- 0.90) and amylase (8.30% +/- 0.42). Only 10 microM of the SP receptor antagonist CP-96,345 significantly inhibited SP-evoked secretion. D-Pro2-D-Phe7-D-Trp9-SP (Ia), D-Pro2-D-Trp7,9-SP (Ib), D-Arg1-D-Trp7,9-D-Leu11-SP (Ic), or 1 microM CP-96,345 were not effective, suggesting that the SP receptor of von Ebner's gland might be an isoform. Propranolol and timolol, beta 1/beta 2-adrenergic receptor antagonists were not effective and the cholinergic receptor antagonist, atropine, was effective in only slightly reducing amylase secretion but not lingual lipase. Differential secretion of the two enzymes was observed for basal and stimulated secretion. Thus, exocytosis may not be the only pathway involved in SP-evoked protein secretion.

show abstract

Transport of proteins across membranes — a paradigm in transition

Cited by 22 publications

References 155 publications

Reconstitution of a chloroplast protein import channel

Reconstitution of a chloroplast protein import channel

Energy-transducing thylakoid membranes remain highly impermeable to ions during protein translocation

Effect of Substance P and Receptor Antagonists on Secretion of Lingual Lipase and Amylase from Rat von Ebner's Gland

Contact Info

Product

Resources

About