腺苷后处理对缺血再灌注心肌细胞凋亡的影响

目的 研究腺苷后处理对大鼠心肌缺血再灌注心肌细胞凋亡及Bcl2和Bax蛋白的影响.方法 32只Wistar大鼠随机分为4组(每组8只):假手术组(Ⅰ组):开胸只穿线不结扎血管,麻醉维持150 min;缺血再灌注组(I/R组):结扎30 min,再灌注120 min;腺苷预处理组(Ⅱ组):缺血前5 min,股静脉缓慢输注腺苷305 μg/(kg* min),持续5 min,而后再按I/R组操作;腺苷后处理组(Ⅲ组):缺血30 min后,股静脉缓慢滴注腺苷305 μg/(kg* min),持续5 min,而后再灌注120 min.采用TUNEL法观察各组心肌细胞凋亡,应用免疫组化SP法观察 Bcl-2和Bax蛋白的表达.结果 与I/R组比较,Ⅱ组及Ⅲ组心肌梗死范围均明显减小,心肌细胞凋亡率降低(P＜0.05),Bax表达水平明显降低,Bcl-2表达水平明显升高(P＜0.05),而Ⅱ组和Ⅲ组间差异无统计学意义.结论 腺苷后处理可以减轻再灌注心肌细胞的凋亡,这一过程可能是通过上调Bcl-2表达和下调Bax表达而实现的.     

红花注射液对肺缺血再灌注损伤时细胞凋亡及Bcl-2和Bax基因的影响

目的探讨细胞凋亡与肺缺血再灌注损伤的关系以及红花注射液的干预及机制。方法健康日本大耳白兔84只,随机分为对照组、缺血再灌注1、3、5h组和红花干预1、3、5h组。复制在体肺缺血再灌注损伤模型。采用电镜和原位缺口末端标记法观测肺组织细胞凋亡情况,并计算凋亡指数;免疫组织化学和原位杂交技术检测各组肺组织Bcl-2和Bax基因表达的变化。结果缺血再灌注组肺组织细胞凋亡指数和Bcl-2、Bax蛋白及mRNA均显著高于对照组(P<0.01)。红花干预组肺组织凋亡指数、Bax蛋白及Bax mRNA低于缺血再灌注组,而Bcl-2蛋白、Bcl-2mRNA以及Bcl-2与Bax的比值较缺血再灌注组上调(P<0.01或P<0.05)。电镜观察发现,缺血再灌注组肺毛细血管内皮细胞和Ⅱ型肺泡上皮细胞超微结构损伤明显,红花干预组损伤明显减轻。肺组织凋亡指数与Bax蛋白、Bax mRNA呈显著正相关(r分别=0.926,0.913;均P<0.01),与Bcl-2/Bax蛋白、Bcl-2/Bax mRNA的比值呈负相关(r分别=-0.367,-0.375;均P<0.01)。结论肺组织细胞凋亡参与了肺缺血再灌注损伤的发生,红花注射液可能通过下调Bax基因的表达,提高Bcl-2/Bax的比值抑制肺组织细胞凋亡,从而减轻肺缺血再灌注损伤。     

人参皂甙Rb1通过促肺泡细胞线粒体自噬以抑制凋亡在大鼠肺气肿模型治疗的机制

目的观察人参皂甙Rb1(GS-Rb1)对大鼠肺气肿的治疗作用是否与自噬的的促进及对凋亡的抑制相关。方法选取40只体重150~200克的SD雄性大鼠随机分为4组,每组10只,以气管内注射猪胰蛋白酶(PPE)建立肺气肿型,腹腔注射给予GS-Rb1及自噬抑制剂氯喹(CLQ)。具体分组为:①阴性对照(Control)组、②治疗(PPE+GS-Rb1)组、③阻断(PPE+GS-Rb1+CLQ)组、④肺气肿组(PPE)组。留取支气管肺泡灌洗液(Bronchoalveolar lavage fluids,BALF)测定其中白细胞数量,采用酶联免疫荧光(ELFA)检测神经元氧化应激因子NAPDH氧化酶活性的变化,同时取肺泡组织,HE染色观察病理学变化、蛋白免疫印记法(WB)测定自噬相关蛋白LC3-Ⅱ、LC3-Ⅰ、P62,线粒体功能蛋白PGC-1a、mtTFA及凋亡相关蛋白Cyto C、Bax、Bcl-2的变化。结果实验结果显示,治疗组较肺气肿组的肺泡组织病理学损伤程度明显好转,NAPDH氧化酶活性及BALF中白细胞总数、中性粒细胞百分比明显下降(P<0.05),同时治疗组相对于肺气肿组的自噬相关蛋白LC3-Ⅱ/LC3-Ⅰ比例升高、P62表达下降,线粒体功能蛋白PGC-1a、mtTFA上调,凋亡促进因子Cyto C及Bax表达明显降低,而凋亡抑制因子Bcl-2表达则明显升高(P<0.05)。采用自噬抑制剂CLQ可明显抑制上述GS-Rb1通过自噬抑制凋亡从而缓解慢阻肺的病理改变、下调BALF中细胞数、中性粒细胞百分比及NAPDH氧化酶活性的功效,(P<0.05)。结论GS-Rb1可能通过促进肺泡细胞线粒体自噬而抑制凋亡途径进而抑制损伤性因子的产生,以达到对肺气肿的治疗作用。     

螺内酯和缬沙坦对高血压大鼠心肌细胞凋亡相关蛋白的影响

目的探讨螺内酯和缬沙坦对肾血管性高血压大鼠左心室细胞凋亡调控相关蛋白P53、Bax及Bcl-2表达的影响。方法选取SD大鼠46只制成肾血管性高血压大鼠模型后,随机分为高血压组(N组,12只)、缬沙坦组(V组,11只)、螺内酯组(S组,12只)、螺内酯加缬沙坦组(S+V组,11只),另选10只未造模大鼠为假手术组(C组),各组治疗10周后行超声心动图检查,测定大鼠颈动脉压,应用免疫组织化学法检测P53、Bax及Bcl-2蛋白的表达。结果治疗10周后,与C组比较,N组大鼠收缩压明显升高(P0.01);与N组比较,V组、S+V组大鼠收缩压明显降低(P0.05);与C组比较,N组大鼠Bax、Bcl-2和Bax/Bcl-2明显升高,S组大鼠Bax和Bax/Bcl-2明显升高(P0.05,P0.01);与N组比较,S组大鼠Bax、Bax/Bcl-2和V组、S+V组大鼠Bax、Bcl-2、Bax/Bcl-2均明显下降(P0.05,P0.01);与S组比较,V组、S+V组大鼠Bax、Bax/Bcl-2明显下降(P0.05,P0.01)。结论两肾一夹肾血管性高血压大鼠左心室肥厚形成过程中,Bax过表达不完全依赖P53调节。缬沙坦可以通过与血管紧张素Ⅱ1型受体结合,抑制心肌细胞凋亡相关蛋白P53、Bax及Bcl-2的表达及左心室肥厚的发生。螺内酯可以通过与醛固酮受体结合明显降低P53及Bax的表达,降低Bax/Bcl-2比值,部分地逆转左心室肥厚。     

艾塞那肽对糖尿病大鼠心肌组织Bcl-2、Bax表达的影响

目的 观察艾塞那肽（EX）对糖尿病（DM）大鼠心肌组织Bcl-2、Bax表达的影响.方法 高糖高脂饲料喂养联合尾静脉注射小剂量链脲佐菌素建立DM大鼠模型20只,随机分为DM组、EX组,EX组皮下注射EX 1.0nmol/（kg·d）,设正常对照10只（N组）.12周后,应用实时定量PCR、Western blot法检测大鼠心肌组织中的Bcl-2、Bax.结果 EX组大鼠空腹血糖（FBG）、糖化血红蛋白（HbA1c）与DM组比较,P均＞0.05.与N组比较,DM组大鼠心肌Bcl-2 mRNA和蛋白表达降低（P均≤0.01）,Bax mRNA和蛋白表达升高（P均≤0.01）,且Bcl-2与BaxmRNA和蛋白表达比值下降（P均≤0.01）;EX组较DM组大鼠心肌Bcl-2 mRNA和蛋白表达升高（P均≤0.01）,BaxmRNA和蛋白表达降低（P均≤0.01）,Bcl-2与Bax mRNA和蛋白表达比值增加（P均≤0.01）.结论 艾塞那肽可能通过上调DM大鼠心肌组织Bcl-2表达、下调Bax表达,从而抑制心肌细胞凋亡,延缓DM心肌病变的发生、发展.     

大鼠脑缺血半暗带神经元蛋白表达和凋亡的动态变化研究

目的观察大鼠局灶性脑缺血再灌注后缺血半暗带神经元蛋白表达和凋亡的动态变化。方法 48只雄性Wistar大鼠随机分为假手术组8只和模型组40只,模型组又根据缺血再灌注时间分为6、24、48 h和3、5 d 5个时间点,各时间点8只。免疫组织化学法观察各组大鼠脑缺血半暗带神经元Bcl-2及Bax蛋白表达,TUNEL法观察相应区域神经元凋亡的动态变化。结果与假手术组比较,模型组大鼠缺血再灌注6 h Bcl-2、Bax阳性细胞明显增加,随时间延长,Bcl-2阳性细胞逐渐下降,48 h最低;而Bax阳性细胞则逐渐增加,于48 h达到高峰;Bcl- 2/Bax比值变化趋势与Bcl-2阳性细胞相一致;各时间点均可见神经元凋亡,48 h达高峰(P<0.01)。相关分析显示,神经元凋亡与Bax蛋白表达呈正相关(r=0.352,P<0.05),与Bcl -2和Bcl-2/Bax比值呈负相关(r=-0.517,r=-0.529,P<0.01)。结论大鼠脑缺血半暗带存在大量神经元凋亡,其机制可能与Bcl-2/Bax比值失衡有关。     

缺血再灌注诱导心肌细胞凋亡及凋亡相关基因表达的研究

目的研究缺血再灌注诱导体外培养大鼠心肌细胞凋亡及凋亡相关基因表达.方法采用体外培养新生大鼠心肌细胞,随机分为正常对照组(Ⅰ组)、模拟缺血2h组(Ⅱ组)、缺血2h后再灌注1h组(Ⅲ组)以及持续缺血3h组(Ⅳ组).TUNEL法检测心肌细胞凋亡,荧光显微镜观察心肌细胞凋亡的形态学特征、免疫组织化学法检测Bcl-2/Bax基因表达.结果心肌细胞I/R后,TUNEL法检测到阳性凋亡细胞,且持续缺血3h组与再灌注组凋亡指数明显高于缺血2h组.荧光显微镜观察到典型的细胞凋亡超微结构;免疫组织化学检测发现Bcl-2蛋白表达下调,Bax蛋白表达上调.结论缺血和缺血再灌注均能诱导心肌细胞凋亡,且心肌细胞凋亡与Bcl-2/Bax基因表达有密切关系.     

加贝酯预防急性胰腺炎的作用机制研究

目的 通过观察加贝酯对胰腺细胞凋亡及Bax、Bcl-2蛋白表达的影响,探讨加贝酯预防大鼠胰管注射法诱导的急性胰腺炎(AP)的相关机制.方法 16只SD大鼠随机分为假手术组(4只)、AP组和加贝酯治疗组(各6只).以50 mmHg(1 mmHg = 0.133 kPa)的恒压向胰胆管内注入30%泛影葡胺诱导SD大鼠AP模型,制模前15 ～ 20 min加贝酯(4 mg·h-1·kg-1体重)静脉持续滴注60 min进行预防.组织病理检查观察胰腺炎症程度,应用TUNEL染色、免疫组化检测胰腺细胞凋亡和Bcl-2、Bax蛋白表达.结果 加贝酯治疗组的胰腺组织病理改变较AP组减轻(P ＜ 0.05).治疗组凋亡指数(AI)、Bax和Bcl-2表达值分别为8.00 ± 1.80,10.12 ± 1.52和1.83 ± 0.39,前两者较AP组显著增高,而Bcl-2蛋白无显著差别.治疗组AI、Bax表达与胰腺的炎症程度呈负相关.结论 加贝酯静脉滴注对大鼠胰管注射法诱导的AP有一定的预防作用.其机制可能与促进细胞凋亡和Bax蛋白表达上调有关.     

原花青素对创伤性脑损伤大鼠大脑皮质Bcl-2、Bax蛋白表达的影响

目的观察原花青素对创伤性脑损伤大鼠大脑皮质抑凋亡基因蛋白Bcl-2和促凋亡基因蛋白Bax表达的影响。方法采用自由落体撞击伤方法制作创伤性脑损伤模型,致伤后连续腹腔注射原花青素1次/d,3 d后用免疫组织化学SABC法染色测定脑损伤大鼠大脑皮层每高倍镜视野Bcl-2和Bax表达阳性细胞数。结果创伤性脑损伤后大脑皮层脑组织Bcl-2和Bax表达阳性细胞数都增加。原花青素处理后能Bcl-2表达阳性细胞数显著增加,Bax表达阳性细胞数显著降低（P均〈0.05）。结论原花青素可提高创伤性脑损伤大鼠大脑皮质Bcl-2表达,抑制Bax表达。     

细胞分裂周期蛋白42在大鼠乳鼠心肌细胞缺血再灌注中的作用

目的:研究细胞分裂周期蛋白42(cell division cycle42,Cdc42)在大鼠乳鼠心肌细胞缺血/再灌注(I/R)中的作用及其可能的作用机制。方法:分离培养SD大鼠心肌细胞,建立I/R模型。实验分组:①正常对照组;②I/R组;③Oligofectamine脂质体组(脂质体组);④脂质体+反义寡脱氧核苷酸组(As组);⑤脂质体+错义寡脱氧核苷酸组(Ms组);⑥C-Jun氨基末端激酶(JNK)阻断剂(SP600125)+脂质体+As组(SP600125+As组);⑦SP600125+脂质体+Ms组(SP600125+Ms组)。用流式细胞仪测定细胞凋亡率,Westernblot测定Cdc42、JNK、p-JNK、Bax、Bcl-2的蛋白含量。结果:I/R组与正常对照组相比较,Cdc42的表达、心肌细胞凋亡率和JNK的磷酸化程度增高,Bcl-2/Bax降低;As组Cdc42的表达、心肌细胞凋亡率和JNK磷酸化程度低于I/R组、脂质体组和Ms组,且Bcl-2/Bax的比值在这4组中是最高的,Cdc42的表达、心肌细胞凋亡率、JNK磷酸化程度和Bcl-2/Bax的比值在I/R组、脂质体组和Ms组之间差异无统计学意义;SP600125+As组较As组、SP600125+Ms组较Ms组JNK的磷酸化程度降低,SP600125+As组与SP600125+Ms组JNK磷酸化程度比较差异无统计学意义。结论:Cdc42在I/R中可促进心肌细胞凋亡,而As可降低I/R中心肌细胞凋亡率,Cdc42在I/R中促进细胞凋亡通过JNK、Bcl-2和Bax信号通路起作用。     

A thermodynamic definition of protein domains

Protein domains are conspicuous structural units in globular proteins, and their identification has been a topic of intense biochemical interest dating back to the earliest crystal structures. Numerous disparate domain identification algorithms have been proposed, all involving some combination of visual intuition and/or structure-based decomposition. Instead, we present a rigorous, thermodynamically-based approach that redefines domains as cooperative chain segments. In greater detail, most small proteins fold with high cooperativity, meaning that the equilibrium population is dominated by completely folded and completely unfolded molecules, with a negligible subpopulation of partially folded intermediates. Here, we redefine structural domains in thermodynamic terms as cooperative folding units, based on m-values, which measure the cooperativity of a protein or its substructures. In our analysis, a domain is equated to a contiguous segment of the folded protein whose m-value is largely unaffected when that segment is excised from its parent structure. Defined in this way, a domain is a self-contained cooperative unit; i.e., its cooperativity depends primarily upon intrasegment interactions, not intersegment interactions. Implementing this concept computationally, the domains in a large representative set of proteins were identified; all exhibit consistency with experimental findings. Specifically, our domain divisions correspond to the experimentally determined equilibrium folding intermediates in a set of nine proteins. The approach was also proofed against a representative set of 71 additional proteins, again with confirmatory results. Our reframed interpretation of a protein domain transforms an indeterminate structural phenomenon into a quantifiable molecular property grounded in solution thermodynamics.     

In vivo protein stabilization based on fragment complementation and a split GFP system

Protein stabilization was achieved through in vivo screening based on the thermodynamic linkage between protein folding and fragment complementation. The split GFP system was found suitable to derive protein variants with enhanced stability due to the correlation between effects of mutations on the stability of the intact chain and the effects of the same mutations on the affinity between fragments of the chain. PGB1 mutants with higher affinity between fragments 1 to 40 and 41 to 56 were obtained by in vivo screening of a library of the 1 to 40 fragments against wild-type 41 to 56 fragments. Colonies were ranked based on the intensity of green fluorescence emerging from assembly and folding of the fused GFP fragments. The DNA from the brightest fluorescent colonies was sequenced, and intact mutant PGB1s corresponding to the top three sequences were expressed, purified, and analyzed for stability toward thermal denaturation. The protein sequence derived from the top fluorescent colony was found to yield a 12 °C increase in the thermal denaturation midpoint and a free energy of stabilization of -8.7 kJ/mol at 25 °C. The stability rank order of the three mutant proteins follows the fluorescence rank order in the split GFP system. The variants are stabilized through increased hydrophobic effect, which raises the free energy of the unfolded more than the folded state; as well as substitutions, which lower the free energy of the folded more than the unfolded state; optimized van der Waals interactions; helix stabilization; improved hydrogen bonding network; and reduced electrostatic repulsion in the folded state.     

Experimental support for the evolution of symmetric protein architecture from a simple peptide motif

The majority of protein architectures exhibit elements of structural symmetry, and "gene duplication and fusion" is the evolutionary mechanism generally hypothesized to be responsible for their emergence from simple peptide motifs. Despite the central importance of the gene duplication and fusion hypothesis, experimental support for a plausible evolutionary pathway for a specific protein architecture has yet to be effectively demonstrated. To address this question, a unique "top-down symmetric deconstruction" strategy was utilized to successfully identify a simple peptide motif capable of recapitulating, via gene duplication and fusion processes, a symmetric protein architecture (the threefold symmetric β-trefoil fold). The folding properties of intermediary forms in this deconstruction agree precisely with a previously proposed "conserved architecture" model for symmetric protein evolution. Furthermore, a route through foldable sequence-space between the simple peptide motif and extant protein fold is demonstrated. These results provide compelling experimental support for a plausible evolutionary pathway of symmetric protein architecture via gene duplication and fusion processes.     

Membrane protein thermodynamic stability may serve as the energy sink for sorting in the periplasm

Thermodynamic stabilities are pivotal for understanding structure–function relationships of proteins, and yet such determinations are rare for membrane proteins. Moreover, the few measurements that are available have been conducted under very different experimental conditions, which compromises a straightforward extraction of physical principles underlying stability differences. Here, we have overcome this obstacle and provided structure–stability comparisons for multiple membrane proteins. This was enabled by measurements of the free energies of folding and the m values for the transmembrane proteins PhoP/PhoQ-activated gene product (PagP) and outer membrane protein W (OmpW) from Escherichia coli. Our data were collected in the same lipid bilayer and buffer system we previously used to determine those parameters for E. coli outer membrane phospholipase A (OmpLA). Biophysically, our results suggest that the stabilities of these proteins are strongly correlated to the water-to-bilayer transfer free energy of the lipid-facing residues in their transmembrane regions. We further discovered that the sensitivities of these membrane proteins to chemical denaturation, as judged by their m values, was consistent with that previously observed for water-soluble proteins having comparable differences in solvent exposure between their folded and unfolded states. From a biological perspective, our findings suggest that the folding free energies for these membrane proteins may be the thermodynamic sink that establishes an energy gradient across the periplasm, thus driving their sorting by chaperones to the outer membranes in living bacteria. Binding free energies of these outer membrane proteins with periplasmic chaperones support this energy sink hypothesis.     

Fibrillation precursor of superoxide dismutase 1 revealed by gradual tuning of the protein-folding equilibrium

Although superoxide dismutase 1 (SOD1) stands out as a relatively soluble protein in vitro, it can be made to fibrillate by mechanical agitation. The mechanism of this fibrillation process is yet poorly understood, but attains considerable interest due to SOD1’s involvement in the neurodegenerative disease amyotrophic lateral sclerosis (ALS). In this study, we map out the apoSOD1 fibrillation process from how it competes with the global folding events at increasing concentrations of urea: We determine how the fibrillation lag time (τ_lag) and maximum growth rate (ν_max) depend on gradual titration of the folding equilibrium, from the native to the unfolded state. The results show that the agitation-induced fibrillation of apoSOD1 uses globally unfolded precursors and relies on fragmentation-assisted growth. Mutational screening and fibrillation m-values (∂ log τ_lag/∂[urea] and ∂ log ν_max/∂[urea]) indicate moreover that the fibrillation pathway proceeds via a diffusely bound transient complex that responds to the global physiochemical properties of the SOD1 sequence. Fibrillation of apoSOD1, as it bifurcates from the denatured ensemble, seems thus mechanistically analogous to that of disordered peptides, save the competing folding transition to the native state. Finally, we examine by comparison with in vivo data to what extent this mode of fibrillation, originating from selective amplification of mechanically brittle aggregates by sample agitation, captures the mechanism of pathological SOD1 aggregation in ALS.     

Pancreatic stone protein – sepsis and the riddles of the exocrine pancreas

Pancreatic stone protein (PSP), discovered in the 1970ies, was first associated with stone formation during chronic pancreatitis. Later, the same protein was independently detected in islet preparations and named regenerating protein 1 (REG1). Additional isoforms of PSP, including pancreatitis-associated protein (PAP), belong to the same protein family. Although the names indicate a potential function in stone formation or islet regeneration, involvements in cellular processes were only suggestive and never unequivocally proven. We established an association between PSP levels in patient blood samples and the development of sepsis. In this review, written in connection with receiving the Lifetime Achievement Award of the European Pancreatic Club, the evolution of the sepsis aspect of PSP is described. We conclude that the true functional properties of this fascinating pancreatic protein still remain an enigma.     

Computing protein stabilities from their chain lengths

New amino acid sequences of proteins are being learned at a rapid rate, thanks to modern genomics. The native structures and functions of those proteins can often be inferred using bioinformatics methods. We show here that it is also possible to infer the stabilities and thermal folding properties of proteins, given only simple genomics information: the chain length and the numbers of charged side chains. In particular, our model predicts ΔH(T), ΔS(T), ΔC_p, and ΔF(T) —the folding enthalpy, entropy, heat capacity, and free energy—as functions of temperature T; the denaturant m values in guanidine and urea; the pH-temperature-salt phase diagrams, and the energy of confinement F(s) of the protein inside a cavity of radius s. All combinations of these phase equilibria can also then be computed from that information. As one illustration, we compute the pH and salt conditions that would denature a protein inside a small confined cavity. Because the model is analytical, it is computationally efficient enough that it could be used to automatically annotate whole proteomes with protein stability information.     

Folding without charges

Surface charges of proteins have in several cases been found to function as “structural gatekeepers,” which avoid unwanted interactions by negative design, for example, in the control of protein aggregation and binding. The question is then if side-chain charges, due to their desolvation penalties, play a corresponding role in protein folding by avoiding competing, misfolded traps? To find out, we removed all 32 side-chain charges from the 101-residue protein S6 from Thermus thermophilus. The results show that the charge-depleted S6 variant not only retains its native structure and cooperative folding transition, but folds also faster than the wild-type protein. In addition, charge removal unleashes pronounced aggregation on longer timescales. S6 provides thus an example where the bias toward native contacts of a naturally evolved protein sequence is independent of charges, and point at a fundamental difference in the codes for folding and intermolecular interaction: specificity in folding is governed primarily by hydrophobic packing and hydrogen bonding, whereas solubility and binding relies critically on the interplay of side-chain charges.     

Method to measure strong protein-protein interactions in lipid bilayers using a steric trap

Measuring high affinity protein-protein interactions in membranes is extremely challenging because there are limitations to how far the interacting components can be diluted in bilayers. Here we show that a steric trap can be employed for stable membrane interactions. We couple dissociation to a competitive binding event so that dissociation can be driven by increasing the affinity or concentration of the competitor. The steric trap design used here links monovalent streptavidin binding to dissociation of biotinylated partners. Application of the steric trap method to the well-characterized glycophorin A transmembrane helix (GpATM) reveals a dimer that is dramatically stabilized by 4-5 kcal/mol in palmitoyloleoylphosphatidylcholine bilayers compared to detergent. We also find larger effects of mutations at the dimer interface in bilayers compared to detergent suggesting that the dimer is more organized in a membrane environment. The high affinity we measure for GpATM in bilayers indicates that a membrane vesicle many orders of magnitude larger than a bacterial cell would be required to measure the dissociation constant using traditional dilution methods. Thus, steric trapping can open new biological systems to experimental scrutiny in natural bilayer environments.     

Retro-translocation of mitochondrial intermembrane space proteins

The content of mitochondrial proteome is maintained through two highly dynamic processes, the influx of newly synthesized proteins from the cytosol and the protein degradation. Mitochondrial proteins are targeted to the intermembrane space by the mitochondrial intermembrane space assembly pathway that couples their import and oxidative folding. The folding trap was proposed to be a driving mechanism for the mitochondrial accumulation of these proteins. Whether the reverse movement of unfolded proteins to the cytosol occurs across the intact outer membrane is unknown. We found that reduced, conformationally destabilized proteins are released from mitochondria in a size-limited manner. We identified the general import pore protein Tom40 as an escape gate. We propose that the mitochondrial proteome is not only regulated by the import and degradation of proteins but also by their retro-translocation to the external cytosolic location. Thus, protein release is a mechanism that contributes to the mitochondrial proteome surveillance.Mitochondrial biogenesis is essential for eukaryotic cells. Because most mitochondrial proteins originate in the cytosol, mitochondria had to develop a protein import system. Given the complex architecture of these organelles, with two membranes and two aqueous compartments, protein import and sorting require the cooperation of several pathways. The main entry gate for precursor proteins is the translocase of the outer mitochondrial membrane (TOM) complex. Upon entering mitochondria, proteins are routed to different sorting machineries (1–5).Reaching the final location is one step in the maturation of mitochondrial proteins that must be accompanied by their proper folding. The mitochondrial intermembrane space assembly (MIA) pathway for intermembrane space (IMS) proteins illustrates the importance of coupling these processes because this pathway links protein import with oxidative folding (6–10). Upon protein synthesis in the cytosol, the cysteine residues of IMS proteins remain in a reduced state, owing to the reducing properties of the cytosolic environment (11, 12). After entering the TOM channel, precursor proteins are specifically recognized by Mia40 protein, and their cysteine residues are oxidized through the cooperative action of Mia40 and Erv1 proteins (7, 13–17). Mia40 is a receptor, folding catalyst, and disulfide carrier, and the Erv1 protein serves as a sulfhydryl oxidase. The oxidative folding is believed to provide a trapping mechanism that prevents the escape of proteins from the IMS back to the cytosol (10, 13, 18). Our initial result raised a possibility that the reverse process can also occur, as we observed the relocation of in vitro imported Tim8 from mitochondria to the incubation buffer (13). Thus, we sought to establish whether and how this process can proceed in the presence of the intact outer membrane (OM). Our study provides, to our knowledge, the first characterization of the mitochondrial protein retro-translocation. The protein retro-translocation serves as a regulatory and quality control mechanism for the mitochondrial IMS proteome.