BioVector® pET-32M3C (pET-32M-3C) 原核高效融合表达质粒载体

BioVector® pET-32M3C (pET-32M-3C) Prokaryotic High-Level Fusion Expression Plasmid Vector

第一部分 中文说明

一 产品基本信息与设计背景

• 载体名称：pET-32M3C（常写作 pET32M3C 或 pET-32M-3C）

• 载体类型：大肠杆菌（E. coli）高丰度、可溶性、原核融合表达质粒。

• 骨架起源与工程化改良：

  • 传统骨架基础：基于经典的 pET-32a(+) 系列骨架优化缩小而来。

  • 3C 酶切位点改良：传统的 pET-32a(+) 载体使用的是肠激酶（Enterokinase, Ek）或 凝血酶（Thrombin） 剪切位点。肠激酶价格昂贵且在非特异性位点极易发生漏切、错切。pET-32M3C 通过分子克隆技术，将纯化标签下游的剪切位点特异性替换为 HRV 3C 蛋白酶（人鼻病毒 3C 蛋白酶 / PreScission Protease） 识别序列。HRV 3C 蛋白酶在 4 摄氏度下即可表现出极高的剪切特异性与活性，能确保融合标签被绝对精准、无痕地切除。

  • M 缩减版设计（pET-32M）：去除了原骨架中一些不必要的非编码冗余片段，使质粒分子量由原先的约 5.9 kb 优化缩减至约 5.8 kb，进一步提高了大片段基因克隆的成功率以及转化效率。

• 复制子与抗性：pBR322 复制 origin（中低拷贝数，以维持强 T7 启动子下的质粒稳定性）；带有 氨苄青霉素抗性基因（AmpR / Amplicillin）。

• 生物安全级别：1级（BSL-1）。

二 核心功能元件与转录/翻译图谱

pET-32M3C 载体专门用于解决结构复杂、易形成不溶性包涵体（Inclusion bodies）的顽固目的蛋白（GOI）的原核表达问题。其核心功能元件的排列和级联图谱如下：

T7 启动子 ── lac 操纵子 ── 强 RBS ── TrxA 融合标签 ── 6×His 标签 ── HRV 3C 酶切位点 ── MCS ── 6×His 标签 (C端) ── T7 终止子

1. 强力的 T7lac 转录轴：

   • T7 启动子（T7 Promoter）：专门识别 T7 RNA 聚合酶，驱动极高速度的转录。

   • lac 操纵子（lac operator）：紧跟启动子下方，通过结合 LacI 阻断未诱导状态下的基础漏表达（Leaky expression），在加入 IPTG 后解除抑制。

2. 高效助溶与纯化双标签系统（Dual-Tag Framework）：

   • TrxA 标签（Thioredoxin, 大肠杆菌硫氧还蛋白，~11.9 kDa）：这是 pET-32 系列的核心王牌。TrxA 是一种高度可溶的内源性蛋白，作为融合伴侣（Fusion partner），它能够极大地促进下游目的蛋白在胞内的正确折叠，强行将原本极易形成不溶性包涵体的毒性/硬核蛋白转化为活性、高丰度的可溶性状态（Soluble fraction）。

   • 6×His 标签（六组氨酸纯化标签）：在 TrxA 下方及多克隆位点（MCS）的 C 端各内置一个 6×His 标签。无论是全长融合表达还是标签切除前后，都能配合固定化金属亲和层析（IMAC，如 Ni-NTA 磁珠/填料）进行高纯度的捕获和精制。

3. 精准剪切中枢（HRV 3C Protease Cleavage Site）：

   • 识别氨基酸核心序列为：Leu-Glu-Val-Leu-Phe-Gln ↓ Gly-Pro。

   • 3C 蛋白酶在 Gln（谷氨酰胺）和 Gly（甘氨酸）残基之间特异性切断。这种剪切往往在 MCS 克隆位点的最前端释放出目标蛋白，避免在切除 TrxA 标签后目的蛋白 N 端残留冗余的无用氨基酸疤痕（Scar residues）。

三 实验室标准转化、表达诱导与蛋白层析纯化步骤

1. 克隆与质粒转化（Cloning & Transformation）：

   • 将目的基因（GOI）通过常规酶切连接或重组克隆技术插入多克隆位点（MCS）中。注意保持读码框（Reading Frame）与前端 TrxA/3C 位点的完全一致。

   • 克隆菌株：连接产物首先转化至 DH5$\alpha$、TOP10 等常规克隆大肠杆菌菌株中进行质粒扩增、测序鉴定。注：克隆菌株不含 T7 RNA 聚合酶，质粒在其中无法进行目标蛋白的表达。

2. 宿主菌诱导表达（Protein Expression & IPTG Induction）：

   • 将测序正确的质粒转化至含有 T7 表达系统的表达宿主菌（如 BL21(DE3)、Origami 2(DE3) 或 Rosetta(DE3)）中。注：由于 TrxA 标签常用于促进二硫键形成，对于富含二硫键的复杂蛋白，强烈推荐配合使用 Origami 2(DE3) 突变株，其胞内氧还环境能与 TrxA 产生完美的协同助溶效应。

   • 挑取单菌落接种于标准 LB 液体培养基（含 100 $\mu$g/ml 氨苄青霉素），37°C 振荡培养至对数中期（OD600 = 0.6–0.8）。

   • 立即向体系中加入终浓度为 0.1 mM 至 1.0 mM 的 IPTG 启动诱导。为了防止过快的翻译导致折叠速度失衡，建议将温度下调至 16°C – 25°C 低温低速振荡诱导过夜（12–16小时），这能使 TrxA 的助溶效果发挥到极致。

3. 蛋白收获、亲和纯化与标签切除（Purification & Tag Removal）：

   • 一纯（First IMAC）：超声波破碎菌体，离心收集上清（可溶性组分）。将上清液上样至 Ni-NTA 亲和层析柱，利用咪唑（Imidazole）梯度洗脱，收集 TrxA-6xHis-3C-GOI 完整融合蛋白。

   • 透析与 3C 酶切（Cleavage）：将洗脱的融合蛋白透析至 3C 酶切缓冲液中以去除高浓度咪唑。按 1:50 或 1:100 的质量比加入重组 HRV 3C 蛋白酶，置于 4°C 冰箱中反应过夜。

   • 二纯（Reverse IMAC / 反向层析）：酶切反应混合物中包含了已被切下的 TrxA-6xHis 片段、未切尽的融合蛋白、带有 His 标签的 HRV 3C 蛋白酶，以及去除了标签的纯净目的蛋白（GOI）。将该混合物重新通过一次全新的 Ni-NTA 柱，此时标签、残留融合物和酶将被全部死死吸附在柱子上，而不带标签的目标纯净蛋白将直接从流穿液（Flow-through）中流出，纯度可直接达到 95% 以上。

四 核心科研应用方向

1. 结构生物学与 X 射线晶体学/冷冻电镜研究（Structural Biology）：利用 HRV 3C 酶切产生的无痕、无多余残基的天然结构蛋白，是进行蛋白质晶体生长、高分辨率核磁共振（NMR）及冷冻电镜（Cryo-EM）三维构象解析的标准首选骨架。

2. 顽固/难折叠哺乳动物蛋白的原核廉价替代表达：许多来源于哺乳动物的激酶、细胞因子、单链抗体片段（scFv），直接在宿主中表达会发生大面积沉淀。pET-32M3C 通过其强力的 TrxA 折叠增溶屏障，实现了在低成本大肠杆菌体系中对这些硬核蛋白的大规模、高活性可溶性工业化制备。

PART 2 ENGLISH SECTION

I General Information and Design Architecture

• Vector Name: pET-32M3C (Commonly cataloged as pET32M3C or pET-32M-3C)

• Vector Type: Recombinant E. coli high-yield, high-solubility prokaryotic fusion expression plasmid.

• Backbone Matrix and Molecular Streamlining:

  • Foundational Framework: Optimized and condensed directly from the classical pET-32a(+) parental matrix.

  • 3C Protease Cleavage Site Insertion: Legacy pET-32a(+) matrices utilize Enterokinase (Ek) or Thrombin endopeptidase cleavage sites. Enterokinase is structurally delicate, cost-intensive, and prone to non-specific off-target proteolytic degradation. The pET-32M3C vector replaces these domains with a premium Human Rhinovirus 3C (HRV 3C) Protease recognition linker (equivalent to PreScission Protease). The HRV 3C protease executes highly specific, single-site cleavage even at 4°C, assuring absolute target precision and scarless fusion partner excision.

  • M-Series Streamlining (pET-32M): Redundant non-coding nucleotide spans have been enzymatically removed, trimming the total vector circumference from ~5.9 kb down to a compact ~5.8 kb, enhancing transformation efficiency and larger gene insert stability.

• Replicon & Selection Antibiotic: pBR322 origin of replication (low-to-medium copy configuration calibrated to maintain plasmid structural stability under intense T7 transcription forces); carries the Ampicillin resistance gene (AmpR).

• Biosafety Level: BSL-1.

II Core Functional Elements and Expression Map

The pET-32M3C platform is customized to bypass inclusion body blockades of highly complex, hydrophobic, or toxic Genes of Interest (GOIs) inside prokaryotic systems. The linear spatial alignment of its expression cassette reads as follows:

T7 Promoter ── lac Operator ── Strong RBS ── TrxA Tag ── 6×His Tag ── HRV 3C Site ── MCS ── C-terminal 6×His Tag ── T7 Terminator

1. High-Velocity T7lac Transcription Core:

   • T7 Promoter: Exclusively matched to the bacteriophage T7 RNA Polymerase, driving exceptionally high transcription rates.

   • lac Operator: Bound downstream of the promoter, it coordinates tightly with the LacI repressor to suppress basal leaky expression before induction. This blockade is instantly lifted upon the introduction of IPTG.

2. Dual-Tag Solubilization and Affinity Capture Framework:

   • TrxA Fusion Tag (Thioredoxin, E. coli derived, ~11.9 kDa): The foundational asset of the pET-32 paradigm. TrxA operates as an ultra-soluble native molecular partner. When fused N-terminally to the passenger protein, it acts as a powerful chaperone shield, driving correct intracellular protein folding and pulling heavily aggregated, insoluble proteins into the active, soluble fraction.

   • Polyhistidine (6×His) Tracks: Flanked both directly downstream of the TrxA element and at the extreme C-terminus of the Multiple Cloning Site (MCS). This enables standardized immobilized metal affinity chromatography (IMAC, utilizing Ni-NTA resins or magnetic beads) at multiple stages of purification.

3. High-Fidelity Cleavage Junction (HRV 3C Site):

   • Core Recognition Sequence: Leu-Glu-Val-Leu-Phe-Gln ↓ Gly-Pro.

   • The protease cuts precisely between the Gln (Q) and Gly (G) residues. Positioned at the N-terminal boundary of the MCS, this site allows the clean removal of the massive TrxA assembly, preventing the retention of unwanted amino acid residues on the N-terminus of the target protein.

III Standard Protocols for Transformation, Expression Tuning, and Reverse-IMAC Purification

1. Cloning and Target Plasmid Prototyping:

   • Clone the target Gene of Interest (GOI) into designated restriction configurations within the Multiple Cloning Site (MCS). Ensure the insert's open reading frame (ORF) aligns perfectly with the N-terminal TrxA and HRV 3C translation frame.

   • Cloning Strain Proliferation: Transform ligation solutions into standard cloning-grade E. coli host strains like DH5$\alpha$ or TOP10 for sequence validation and high-yield plasmid extraction. Note: Cloning strains lack the T7 RNA Polymerase gene, making target protein expression impossible inside these lines.

2. Expression Host Selection & Metabolic Induction:

   • Deliver verified pET-32M3C constructs into specialized DE3 expression hosts (such as BL21(DE3), Origami 2(DE3), or Rosetta(DE3)). Note: Because TrxA excels at guiding proper disulfide bond creation, complex target proteins with dense disulfide bridges should ideally be expressed in Origami 2(DE3). This host's mutated, oxidized cytoplasmic environment works synergistically with the TrxA tag.

   • Inoculate a verified single colony into standard liquid LB medium containing 100 $\mu$g/mL Ampicillin, shaking at 37°C until it enters mid-log phase (OD600 = 0.6–0.8).

   • Initiate target protein expression by supplementing the culture with 0.1 mM to 1.0 mM IPTG. To maximize soluble protein accumulation and prevent translation crowding, lower the temperature to 16°C–25°C for slow, low-temperature overnight induction (12–16 hours).

3. Cell Harvest, Primary Capture, and Tag Removal Routine:

   • Primary IMAC Capture: Disrupt the cells via sonication and clarify the lysate via high-speed centrifugation to collect the soluble fraction. Load the supernatant onto a Ni-NTA affinity column, and run an imidazole gradient to isolate the intact TrxA-6xHis-3C-GOI fusion product.

   • Dialysis & 3C Protease Cleavage: Dialyze the eluted fraction into standard 3C cleavage buffer to strip away excess imidazole. Supplement the dialyzed protein pool with recombinant His-tagged HRV 3C protease at a mass ratio between 1:50 and 1:100, then incubate at 4°C overnight.

   • Reverse IMAC Polishing: The digested mixture now contains cleaved TrxA-His tags, the His-tagged HRV 3C protease itself, minor uncleaved fusion remnants, and the untagged, pure target protein (GOI). Pass this entire slurry through a fresh Ni-NTA column. All His-tagged components (tags, enzymes, and remnants) will bind to the matrix, while the highly pure, untagged target protein passes directly through into the flow-through fraction, routinely yielding >95% purity.

IV Strategic Research Fields

1. Structural Biology & High-Resolution Macromolecular Analytics: The generation of untagged proteins with native N-terminal sequences makes this vector an ideal tool for producing samples for X-ray crystallography, high-resolution Nuclear Magnetic Resonance (NMR), and Cryo-Electron Microscopy (Cryo-EM) structural determinations.

2. Soluble Prototyping of Challenging Mammalian Targets: Many eukaryotic kinases, signaling cytokines, and single-chain variable fragments (scFvs) form inclusion bodies when expressed in standard bacterial setups. The pET-32M3C vector leverages its built-in TrxA solubility shield to enable low-cost, scalable production of these difficult targets within an E. coli platform.

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