#protein translocation

Protein Sorting and Trafficking

What is the function of the OXA complex in mitochondria?

Evolutionary conservation membrane insertases bacterial plasma membrane reveals OXA family YidC Alb3 Oxa1. Mitochondrial Oxa1 45 kDa inner membrane five transmembrane helices forms co-translational insertion site adjacent matrix mitoribosome large subunit near exit tunnel via Mrpl45 interaction. Several highly hydrophobic proteins encoded mitochondrial DNA including subunit II III cytochrome oxidase Cox2 Cox3 subunit a c ATP synthase Atp6 Atp9 cytochrome b Cob synthesized matrix mitoribosomes because extreme hydrophobicity would hamper import. Oxa1 central hydrophilic groove provides aqueous path lateral gate opening lipid bilayer facilitating insertion nascent helices using positive inside rule proton motive force independent ATP. Nuclear-encoded proteins first fully imported matrix via TIM23 such Cox18 dependent and some ATP synthase subunits also require subsequent export inner membrane conservative sorting analogous Sec-independent insertion again via Oxa1. Oxa1 does not insert beta-barrel outer proteins relying SAM complex Tob55 nor directly drive matrix import via TIM23 nor act proton pump; Complexes I III IV pump protons. Deletion Oxa1 yeast pleiotropic respiratory deficiency failure assembling Complexes IV V loss membrane potential and biogenesis.

Ref: Hennon et al., Front Physiol 2015: OXA insertase inserts proteins into mitochondrial inner membrane.

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Protein Sorting and Trafficking

What is the primary role of BiP in protein translocation?

BiP, also termed GRP78 encoded by HSPA5 gene, is abundant ER lumenal Hsp70 family member exhibiting both holdase and molecular motor activities. Structurally it comprises N-terminal nucleotide binding domain with actin-like ATPase fold and C-terminal substrate binding domain consisting of beta-sandwich that cradles extended peptide and alpha-helical lid that closes over cleft. In ATP-bound open state affinity low, allowing scanning of incoming nascent chains emerging through Sec61. DnaJ proteins like ERdj3/ERdj4 stimulate ATP hydrolysis, converting to ADP-bound closed conformation that clamps onto hydrophobic stretches typically five to seven residues enriched in branched aliphatic and aromatic side chains, preventing retrograde movement and aggregation. Nucleotide exchange factor Sil1 and Grp170 promote ADP release allowing substrate release for folding attempt. As ratchet during post-translational translocation, repetitive BiP binding prevents back-sliding providing directional force. Beyond translocation, BiP assists immunoglobulin folding, retains unassembled subunits, regulates UPR sensors IRE1, PERK, ATF6 via sequestration under resting conditions and releases upon stress. Signal cleavage or pore formation are not its activities, highlighting specialized chaperone motor function.

Ref: Pobre et al., Mol Biol Cell 30: 2019, BiP ATPase Ratchet Preventing Back-Sliding.

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Protein Sorting and Trafficking

Which ATPase is required for post-translational protein translocation into the ER?

While most secretory and membrane proteins in mammals rely on SRP-dependent co-translational targeting, small proteins less than about one hundred residues and many soluble yeast proteins synthesized completely before targeting rely on post-translational translocation requiring distinct chaperone system. Cytosolic Hsp70 homologs Ssa1 maintain substrate unfolded, then Sec62/Sec63 heterodimer together with Sec71/Sec72 accessory proteins in yeast forms heptameric Sec complex around Sec61 channel to recruit substrate. Lumenal Hsp70 BiP, called Kar2 in Saccharomyces, binds DnaJ domain of Sec63 which stimulates its ATPase activity. ATP-bound BiP recruited to incoming chain hydrolyzes ATP to ADP, converting to closed high-affinity conformation clamping onto hydrophobic patches, preventing back-sliding through channel. Nucleotide exchange factors Sil1 and Grp170 promote ADP release for BiP recycling, allowing repeated capture events that rectify Brownian motion into directional pulling force. Each ATP hydrolysis cycle advances chain inward independent of ribosome. SRP, Sec61 alone without BiP and Ran-GTP dependent nuclear import do not provide ATP-driven pulling for this mode, so BiP is essential ATPase converting chemical energy into vectorial movement for post-translational ER entry and secretory pathway maintenance.

Ref: Matlack et al., Cell 97: 1999, BiP and Sec63 in Post-translational Translocation.

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Protein

Which of the following chaperones is most commonly associated with protein transport across organelle membranes?

Hsp70 is the scientifically accurate answer to this question. Within the study of Protein Folding, this concept is well-established through extensive research and is documented in standard scientific literature. The specific properties, mechanisms, or characteristics of Hsp70 directly address what is being asked. Among the other options, Hsp40, Hsp100, and Hsp60 do not correctly answer this question because they either refer to different concepts, describe properties of other molecules or processes, or represent common misconceptions about this topic.

Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 4

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