Author

Bor-Ruei Lin

Date of Award

12-3-2008

Degree Type

Closed Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

First Advisor

Phang C. Tai - Chair

Second Advisor

Parjit Kaur

Third Advisor

Chun Jiang

Abstract

We have developed a novel, sensitive and less time-consuming method to detect activity of the SecA-dependent protein-conducting channels. Nanogram levels of E. coli inverted membrane vesicles were injected into Xenopus oocytes, and ionic currents were recorded using the two-electrode voltage clamp. Currents were observed only in the presence of E. coli SecA in conjunction with E. coli membranes. The observed currents showed outward rectification in the presence of KCl as permeable ions and were significantly enhanced by coinjection with the precursor protein, proOmpA, or active LamB signal peptide. Channel activity was blockable with sodium azide or adenylyl 5’-(β, γ-methylene)-diphosphonate, a non-hydrolyzable ATP analog, both of which are known to inhibit SecA protein activity. Channel activity was also stimulated by oocyte endogenous precursor proteins, which could be inhibited by puromycin. In the presence of puromycin, exogenous proOmpA or LamB signal peptides, but not defective signal peptides, stimulated the ionic currents. We also measured SecA-dependent currents with membranes depleted of SecYEG. Wild-type LamB signal peptides, or precursor proteins stimulated ionic currents following a co-injection of SecYEG¯ membranes with puromycin. Excess exogenous SecA stimulated ionic currents through SecYEG¯ membranes. Similar activities of added SecA were observed with reconstituted membranes depleted of SecYEG. Currents through such SecYEG-depleted membranes were also stimulated by addition of defective LamB signal peptides and unfolded mature PhoA protein. In contrast, currents produced by the membranes containing wild-type SecYEG were not so stimulated, but ionic currents were stimulated through mutant strains, similar to PrlA (SecY) suppressors, e.g. PrlA4, or PrlA665 membranes, suggesting that the proofreading function of SecY was bypassed in these membranes. We have observed that azide can inhibit ionic currents when E. coli wild-type MC4100 membranes were injected with proOmpA or LamB signal peptides into Xenopus oocytes. However, such inhibition was lost when observed with oocyte-endogenous signal peptides in the absence of bacterial signal peptides. Moreover, azide did not show complete inhibition upon using SecYEG¯ membranes or SecYEG¯ reconstituted membranes plus excess SecA in the presence or absence of LamB signal peptides. Such conformational alterations reflect different sensitivity in response to azide during the opening of protein-conducting channels.

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