New version page

Spatial Localization of Chaperone

This preview shows page 1-2-3-19-20-38-39-40 out of 40 pages.

View Full Document
View Full Document

End of preview. Want to read all 40 pages?

Upload your study docs or become a GradeBuddy member to access this document.

View Full Document
Unformatted text preview:

ChapDistribPaper_final_01172011.pdfsupplementary_final_01172011Spatial Localization of Chaperone Distribution inthe Endoplasmic Reticulum of YeastMarc Griesemer1, Carissa Young2, Anne Robinson2, and LindaPetzold11Department of Computer Science, University of California, SantaBarbara, CA 931062Department of Chemical Engineering, University of Delaware,Newark, DE 19716AbstractIn eukaryotes, the endoplasmic reticulum (ER) serves as the first membrane-enclosed organelle in the s ec retory pathway, with functions including proteinfolding, maturation, and transport. Molecular chaperones, of the Hsp70 familyof proteins, participate in assisting these processes and are e ss ential to cellu-lar function and survival. BiP is a resident Hsp70 chaperone in the ER ofSaccharomyces cerevisiae. In this study we have created a partial differentialequation (PDE) model to examine how BiP interacts with the membrane-boundco-chaperone Sec63 in translocation, a process in which BiP assists in guidinga nascent protein into the ER lumen. We found that when Sec63 participatesin translocation through localization at the membrane, the spatial distributionof BiP is inhomogeneous, with more BiP at the surface. When translocationis inhibited through a disabling of Sec63’s membrane tether, the concentrationof BiP throughout the ER becomes homogeneous. Our computational simula-tions suggest that Sec63’s localization and the resulting binding to BiP near themembrane surface of the ER enable a heterogeneous distribution of BiP withinthe ER, and may facilitate BiP’s role in translocation.11 Int roductionMolecular chaperones participate in a wide range of processes essential to cellu-lar function and survival. Found in all organisms, and ubiquitously distributedin the major compartments of eukaryotic cells, most are intricate players in theresponse to cellular stress [1]. Hsp70s assist in protein folding and maturation,assembly or disassembly of complexes, ribosomal RNA processing, translocationof newly synthesized proteins, suppression of aggregation, and protein degrada-tion. The versatility among molecular chaperones is intriguing, for they have asingle purpose: to bind protein substrates. The very high degree of conservationamong Hsp70 proteins may favor a unique molecular mechanism common to all,whereas functional differences may depend on modulating co-chaperones suchas Hsp40s and nucleotide exchange factors (NEFs) [2]. This phenomenon of aprotein having multiple, sometimes competing functions, indicates a high levelof systems control.In the lumen of the endoplasmic reticulum (ER), a network of chaperonesand cofactors ensures the proper folding of secretory proteins. One of the mostabundant proteins of the ER is the Hsp70 molecular chaperone, BiP. Throughbiochemical and genetic exp erime nts, BiP has been identified in critical cel-lular processes including protein translocation of newly synthesized precursorsacross the ER membrane, folding and maturation, karyogamy, and ERAD (ER-associated degradation) where unfolded or abnormally folded proteins are sentback to the cytosol for degradation [3–7]. Like other Hsp70 chaperones, BiPassists the folding of a protein by repeated ATP-controlled cycles of bindingand release. Co-chaperones, such as Hsp40s, stimulate the binding of molec-ular chaperones to the substrate and regulate chaperone activities [8]. In theER of S. cerevisiae, co-chaperone Sec63 directly interacts with BiP, increasingits affinity for the nascent proteins proceeding through the translocation pore(Figure 1). Simultaneously within the ER lumenal environment, co-chaperonesScj1 and Jem1 associate with BiP during the processes of protein maturation2and karyogamy, respectively [6, 7, 9]. BiP, Jem1, and Scj1 are all involved inthe degradation of aberrant soluble proteins through ERAD [10]. As illustratedin Figure 1, Sil1 and Lhs1 are the nucleotide exchange factors (NEF) that playkey roles in these processes by triggering substrate release [11, 12].The regulation of Hs p70-Hsp40-NEF interactions is best understood for Es-cherichia coli homologues DnaK, DnaJ, and GrpE. Mechanistic details havebeen experimentally explored and mathematically modeled [13–19]. Ye t, E.coli is an organism that does not consist of membrane-bound compartmentsto perform distinct cellular functions. Additionally, many chaperone-mediatedprocesses involve spatial aspects, such as the subcellular localization of messen-ger RNA leading to translation of their encoded proteins [20]; subcompartmentsof the nucleus implicated in the processes of transcription and splicing [21, 22];and the spatial localization of BiP at the ER membrane maintaining the per-meability barrier during protein translocation [23].Yeast, S. cerevisiae, is a simple eukaryotic organism that compartmental-izes selective processes and protein-protein interactions to specified organelles.Proteomic studies have verified the location of ER-resident proteins of S. cere-visiae and identified absolute levels of protein expression [24, 25]. These datasuggest that the concentration of BiP exceeds the level of co-chaperones by atleast an order of magnitude at normal growth conditions, and is significantlyup-regulated during quality control mechanisms of the cell including the heatshock response (HSR) and unfolded protein response (UPR) [26, 27]. It is alsoknown that multiple BiPs can bind to substrates with varying affinities [28].Experimentally, the co-chaperone Sec63 must be spatially localized at a sub-organelle level – the ER membrane – in order for translocation to occur [29].Given this evidence, we hypothesize that the spatial localization of BiP andinteractions with co-chaperones regulate its diversity and functionality in theER of S. cerevisiae.The remainder of the paper is organized as follows: in Section 2, we de-scribe our model development. The results of our simulations are pres ented in3Section 3. In Section 4, we examine the sensitivity of our model to parameterperturbations.2 ModelsModelers have attempted to discern the role of Hsp70 chaperones in assist-ing and accelerating translocation of proteins across membranes of organelles,specifically the ER and mitochondria of S. cerevisiae. Previous work has fo-cused on transport mechanisms, including either the Brownian ratchet model[30], comparison to the power stroke model [31, 32], or a unifying mechanism ofboth termed entropic pulling [2]. We have examined the significance of spatialeffects


Loading Unlocking...
Login

Join to view Spatial Localization of Chaperone and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Spatial Localization of Chaperone and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?