Wickner, William

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Dartmouth College Dartmouth Digital Commons Open Dartmouth: Peer-reviewed articles by Dartmouth faculty Faculty Work 10-1995 The Nascent-Polypeptide-Associated Complex: Having a "NAC" for Fidelity in Translocation. William Wickner Dartmouth College Follow this and additional works at: https://digitalcommons.dartmouth.edu/facoa  Part of the Medical Biochemistry Commons Dartmouth Digital Commons Citation Wickner, William, "The Nascent-Polypeptide-Associated Complex: Having a "NAC" for Fidelity in Translocation." (1995). Open Dartmouth: Peer-reviewed articles by Dartmouth faculty. 1618. https://digitalcommons.dartmouth.edu/facoa/1618 This Article is brought to you for free and open access by the Faculty Work at Dartmouth Digital Commons. It has been accepted for inclusion in Open Dartmouth: Peer-reviewed articles by Dartmouth faculty by an authorized administrator of Dartmouth Digital Commons. For more information, please contact dartmouthdigitalcommons@groups.dartmouth.edu. Proc. Natl. Acad. Sci. USAVol. 92, pp. 9433-9434, October 1995CommentaryThe nascent-polypeptide-associated complex: Having a "NAC" forfidelity in translocationWilliam WicknerDepartment of Biochemistry, Dartmouth Medical School, 7200 Vail Building, Hanover, NH 03755-3844Intracellular protein targeting and mem-brane translocation are highly regulatedevents, catalyzed by a cascade of specificinteractions and coordinated with proteinfolding. On pages 9435-9439 of this issueof the Proceedings, Lauring et al. (1) showthat the nascent-polypeptide-associatedcomplex (NAC) plays a prominent role inregulating the association of signal recog-nition particle (SRP) with nascent chainsand in preventing the binding of ribo-somes with nonsecretory nascent chains tothe endoplasmic reticulum. These studies,together with a recent dazzling array ofbiochemical and genetic studies in caninepancreas, yeast, Escherichia coli, and plantchloroplasts, give us added appreciationfor the versatility of SRP function in dif-ferent organisms and circumstances. SRPis not just for cotranslational targetinganymore but may be viewed as a chaper-one and as a GTP-linked system for trans-locational fidelity.SRP was first purified as a translocationfactor from a salt extract of canine micro-somes (2, 3). It binds with micromolaraffinity to ribosomes (2, 3) but with nano-molar affinity to ribosomes with nascentchains bearing a signal sequence. SRPbinds signal sequences via its 54-kDa sub-unit (4,5). Once bound, SRP slows or evenarrests the growth of the nascent chain (3)until the complex binds to the SRP recep-tor of the endoplasmic reticulum (6, 7).This association triggers GTP hydrolysisby SRP and the SRP receptor and SRPdissociation from both the nascent chainand the receptor itself (8-12). The effectsof SRP on chain growth provide a meansfor promoting cotranslational transloca-tion, whereas targeting may be achievedby the GTP-regulated cycle with its recep-tor. However, SRP can function late inchain growth (13, 14) or even with full-length, ribosome-bound chains (15), andits capacity to slow translation is notneeded for in vitro reconstitution of trans-location (16-18).Each aspect of SRP function was firstelucidated by careful biochemistry of themammalian system and then tested in thegenetically tractable translocation reac-tions of Saccharomyces cerevisiae and E.coli. These studies not only have con-firmed the importance of SRP for trans-location but also have shed new light on itsrole. In yeast, SRP and its receptor arerequired for normal growth rates, al-though the cells can adapt to their absence(19-22). Several secreted proteins dependstrongly on SRP for translocation, othersare strongly stimulated by its presence,whereas others apparently translocatenormally in its absence (19).E. coli SRP is also essential for growth(23, 24) but has no evident effect on thesecretion of many proteins and only aminor kinetic effect on others (23-25).This SRP does, however, have a largeeffect on the secretion of pre-,B-lactamase,a well-studied periplasmic enzyme (25,26). Strikingly, pre-13-lactamase is ex-ported entirely posttranslationally (27,28), showing that SRP can function inposttranslational translocation. Neverthe-less, its function may have to begin withshort nascent chains, as indicated in ele-gant crosslinking studies (29). Recentstudies by J. Luirink and colleagues (J.Luirink, personal communication) haveshown that ribosome-bound E. coli SRPand trigger factor compete for nascentchain association in a manner reminiscentof mammalian NAC and SRP (see below).Trigger factor can associate with theGroEL chaperone (0. Kandror, M. Sher-man, and A. L. Goldberg, personal com-munication), and this association cangreatly enhance the capacity of GroEL tobind certain unfolded proteins. Thus, trig-ger factor might deliver nascent cytoplas-mic proteins to this chaperone. E. coli SRPcan associate with the SRP receptor ho-molog FtsY and activate a signal peptide-sensitive GTPase activity (30), but trans-location is believed to proceed via SecAand SecYEG, the E. coli preprotein trans-locase (31).The most striking confirmation of thechaperone role of SRP comes in studies ofchloroplast protein import (32). The nu-clear-encoded precursor form of the light-harvesting chlorophyll a/b protein is syn-thesized on cytoplasmic polysomes andimported across two membranes into thematrix. There, it associates with a complexthat contains the chloroplast homolog ofthe 54-kDa subunit of SRP, which is re-quired for subsequent import of the pre-cursor into the thylakoid. This study, inagreement with other model studies (33,34), clearly establishes the capacity of SRPto support posttranslational translocationin a nonribosomal context.The elegant studies of Wiedmann andcolleagues provide an important piece ofthe puzzle. They have found that NAC isa heterodimeric protein (35), which formsthe shared "exit tunnel" from the ribo-some (36). It competes with SRP for as-sociation with nascent chains; SRP has ahigher affinity for nascent chains with asignal sequence, whereas NAC has a loweraffinity for these same sequences. Theassociation of a nascent chain with NACnot only prevents SRP association, whichmay otherwise lead to mistargeting andaberrant translocation, but also blocks ri-bosome association with the endoplasmicreticulum membrane (1). Indeed, in theabsence of NAC, the system becomes in-dependent of SRP and SRP receptor, andthe affinity of ribosomes for the Sec6lcomplex is sufficient to target a nascentchain to the translocation site (37, 38).Translocation in this case is still stronglyenhanced by a functional signal sequence,suggesting additional steps of SRP-independent signal sequence recognition(38). However, both the Wiedmann andRapoport laboratories have shown thatthe ribosome is a targeting factor of majorimportance and that SRP and NAC reg-ulate this targeting capacity.SRP and NAC may be viewed as aversatile targeting chaperone team. BothSRP and NAC are ribosomally bound andmay compete for signal sequences. SRP,which acts at a particular step of thetranslation cycle (39), has a higher signalsequence binding affinity. Although therole of SRP in mammalian translocation isnot entirely tied to its capacity to slowpolypeptide elongation (16-18), this prop-erty helps to ensure a cotranslational char-acter. Delivery of the nascent chain com-plex to the endoplasmic reticulum mem-brane is blocked by NAC (for non-secretory proteins); it is not yet knownwhether NAC has a direct role in deliver-ing these proteins to cytosolic chaperones.The delivery of presecretory nascent chaincomplexes bearing SRP to the endoplas-mic reticulum membranes may rely onthree binding affinities: that of SRP forthe SRP receptor, that of the ribosome forthe Sec6l complex (40), and that of thesignal sequence itself for the Sec6l com-plex (38) and for lipid (41, 42). The reg-ulation, hierarchy of binding affinities,and temporal order of these three inter-9433Proc. Natl. Acad. Sci. USA 92 (1995)actions are still under study. The interac-tion of SRP with its receptor triggers GTPbinding and hydrolysis and the release ofSRP from both the nascent chain and theSRP receptor (11, 12). NAC adds selec-tivity to the association of SRP with signalsequences and the association of ribo-somes with the Sec6l complex; its discov-ery and characterization by the Wiedmanngroup has added an important dimensionto our understanding of translocation.1. Lauring, B., Kreibich, G. & Wiedmann,M. (1995) Proc. Natl. Acad. Sci. USA 92,9435-9439.2. Walter, P. & Blobel, G. (1980) Proc. Natl.Acad. Sci. USA 77, 7112-7116.3. Walter, P. & Blobel, G. (1981) J. Cell Bio.91, 557-561.4. Krieg, U. C., Walter, P. & Johnson, A. E.(1986) Proc. Natl. Acad. Sci. USA 83,8604-8608.5. Kurzchalia, T. V., Wiedmann, M., Gir-shovich, A. S., Bochkareva, E. S., Bielka,H. & Rapoport, T. A. (1986) Nature (Lon-don) 320, 634-636.6. Gilmore, R., Walter, P. & Blobel, G.(1982) J. Cell Biol. 95, 470-477.7. Meyer, D. I., Krause, E. & Dobberstein,B. 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The Nascent-Polypeptide-Associated Complex: Having a  NAC  for Fidelity in Translocation.

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The Nascent-Polypeptide-Associated Complex: Having a NAC for Fidelity in Translocation.

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