🧬 1. Big Picture: Why Selection Methods?

When you create a library of mutants (e.g. 10⁶–10¹¹ variants), the challenge is:

👉 Find the one protein with the desired property.

Two main strategies:

• Screening → test each individually (slow ❌)
• Selection → only “good” variants survive (fast ✅)

🧪 2. Complementation (Phenotype-Based Selection)

💡 What does “complement phenotype” mean?

It means: 👉 A gene restores a missing function in a cell

Example:

• You delete an essential gene from bacteria
• Provide mutated versions on a plasmid
• Only bacteria with a functional protein survive

👉 That survival = complementation of phenotype

💡 “Use plasmid → active protein survives?”

• Plasmid = carries gene variants
• If variant works → cell survives
• If not → cell dies

✔️ Survival = functional protein ❌ Death = non-functional mutant

🔗 3. Genotype–Phenotype Link (CRITICAL CONCEPT)

💡 Why link genotype and phenotype?

You must connect:

• Phenotype = protein function
• Genotype = DNA sequence

👉 Because only DNA can be amplified

💡 “Fusion protein → link genotype?”

Yes:

• You physically attach your protein to something that carries its DNA
• Example:
  • Virus surface displays protein
  • DNA encoding that protein is inside

✔️ Now you can:

1. Select protein
2. Recover its DNA

🦠 4. Filamentous Phage Display (M13 System)

🧬 What is it?

A virus (phage) that infects bacteria like E. coli

🧱 Structure of M13 Phage

• Long filament shape
• ssDNA genome inside
• Protein coat outside:

Ends:

• Left end → Protein 7 + 9
• Right end → Protein 6 + 3

Body:

• Protein 8 (~2700 copies)

💡 Your question:

“At ends: 7,9 and 6,3?”

✔️ Correct:

• One end: p7 + p9
• Other end: p6 + p3

🧬 Why Protein 3 (pIII) is important

💡 “G3p = infection protein?”

Yes:

👉 Protein III:

• Recognizes F-pilus on bacteria
• Initiates infection

🧫 Host Requirement

💡 “E. coli with pilus?”

✔️ Yes:

• Needs F-pilus
• Without it → no infection

🔄 5. Phage Life Cycle

🧠 Key idea: Non-lytic system

💡 Lytic vs Non-lytic

💡 “Why lytic mentioned?”

Just for comparison — M13 is non-lytic only

💡 “Non-lytic = assembled in membrane?”

Yes:

• Virus assembles while exiting
• It is extruded through membrane
• No accumulation inside cell

🧬 Steps:

1. Recognition
   • pIII binds F-pilus (always at the tip)
2. Depolymerization
   • Pilus shrinks → pulls phage to membrane
3. DNA entry
   • ssDNA enters cell
4. Replication
   • ssDNA → dsDNA (replicative form)

👉 💡 “dsDNA = replicative form?” ✔️ Yes

5. Production
   • New ssDNA generated

👉 💡 “ssDNA covered by pV?” ✔️ Yes (stabilization)

6. Assembly + release
   • Virus exits cell

🧬 6. Protein 8 (pVIII) Issues

💡 “M13 α-helix destroyed?”

Yes:

• p8 = single alpha helix
• Adding fusion protein: ❌ destabilizes structure

🧬 7. Protein 3 Structure

💡 “3 domains with glycine linkers?”

✔️ Correct:

• 3 domains
• Connected by flexible Gly-rich linkers

👉 Glycine = flexible → allows fusion protein attachment

📦 8. Vector Size

💡 “Smaller vector is better?”

Yes:

• Smaller DNA → easier to:
  • transform into cells
  • create large libraries

🔁 9. Phagemid System (Hybrid System)

💡 “Phagemid = plasmid + phage?”

✔️ Exactly:

Contains:

• M13 ori → phage replication
• ColE1 ori → plasmid replication

💡 Why needed?

To fix problems:

• Large genome ❌
• Poor infection ❌
• Avidity issues ❌

🔧 10. Helper Phage Rescue

💡 “Why 2 genomes?”

Inside bacteria:

• Phagemid (your fusion protein DNA)
• Helper phage (full phage genes)

💡 “2 types of pIII?”

✔️ Yes:

• From phagemid → fused protein
• From helper phage → normal protein

Result:

👉 Only 1 fusion pIII per phage → monovalent display

💡 Color confusion:

• Green = phagemid pIII (fusion)
• Brown = helper phage pIII

⚠️ 11. Why M13 Display is NOT good for PTMs

💡 “Not suitable for PTM?”

✔️ Correct:

• Bacteria cannot do:
  • glycosylation
  • many phosphorylations

👉 So protein is not “native”

🍞 12. Yeast Display System

💡 Key idea:

Display protein on yeast surface

💡 “Fusion protein on surface?”

Yes:

• Protein fused to Aga2
• Binds Aga1 in cell wall

👉 Covalent linkage

💡 “Wild-type yeast interaction?”

✔️ Aga1–Aga2 naturally interact

💡 “Yeast diffused by proteins?”

Not diffusion — it means: 👉 Yeast surface is covered with proteins

💡 Why yeast is better for PTMs?

✔️ Because yeast is eukaryotic

• Can perform:
  • glycosylation
  • folding similar to humans

💡 “Cell wall importance?”

• Anchors fusion proteins
• Enables display outside cell

🔬 13. FACS (Fluorescence Activated Cell Sorting)

💡 “How does it work?”

1. Label protein with fluorescent tag
2. Cells pass laser one-by-one
3. Signal measured
4. Cells sorted

💡 “Does it use droplets?”

✔️ Yes:

• Each cell in a droplet
• Charged and sorted

💡 “Excite fluorophore?”

Laser excites → emits light → detected

📊 14. Library Size Limits

💡 “Yeast limited?”

Yes → slower sorting (FACS bottleneck)

🦠 15. Bacterial Display

💡 “Same as yeast?”

Similar concept but:

💡 “Protein on membrane?”

✔️ Yes:

• Display via:
  • outer membrane proteins
  • lipoproteins
  • appendages

🧠 Key Takeaways

• Selection requires genotype–phenotype link
• Phage display = virus-based selection
• Phagemid = optimized hybrid system
• Yeast display = better for PTMs
• FACS = key sorting technology
• Trade-offs:
  • complexity vs biological relevance

🔬 16. Infection Details & Recognition

💡 “Recognition always happens at the end?”

✔️ Yes — experimentally observed.

• Protein III (pIII) binds the tip of the F-pilus
• Not along the side

👉 Why?

• The tip is the active interaction site
• Likely more accessible + correct conformation

💡 “Depolarization needed for phage life cycle?”

This is slightly off.

✔️ What actually happens:

• Depolymerization (not depolarization) of F-pilus

👉 Meaning:

• Pilus shrinks → pulls phage toward cell

❌ Not electrical depolarization ✔️ It is mechanical retraction

🧬 17. DNA Forms & Replication

💡 “dsDNA = replicated form?”

✔️ Yes:

• Incoming DNA = ssDNA
• Inside bacteria → converted to dsDNA (replicative form)

👉 This is:

• Transcribed
• Replicated

💡 “ssDNA coded by pV?”

✔️ Clarification:

• pV does NOT code ssDNA
• It binds and stabilizes ssDNA

👉 Prevents:

• degradation
• premature conversion to dsDNA

🧬 18. Protein Display Constraints

💡 “Alpha helix destroyed?”

✔️ Important nuance:

• pVIII = single α-helix
• Fusion protein adds bulk

👉 Consequence:

• disrupts helix packing
• destabilizes phage coat

💡 Why pIII is preferred:

• Larger
• Flexible domains
• Tolerates fusion proteins

🔗 19. Avidity vs Affinity (Critical Concept)

💡 Problem in phage display:

If multiple copies of protein bind:

👉 Avidity effect

• multiple weak interactions → strong total binding

❌ Problem:

• Masks true affinity differences

💡 Solution:

✔️ Phagemid system → monovalent display

👉 Only 1 fusion protein per phage → measures true affinity

🧬 20. Phagemid System Deep Explanation

💡 “M13 ori?”

• Origin for phage replication
• Enables ssDNA production

💡 “colE1?”

• Origin for plasmid replication
• Works inside bacteria

💡 Why both?

• Dual system:
  • plasmid behavior inside cell
  • phage behavior for display

🔄 21. Rescue Mechanism (Important)

💡 “Will go to bacteria…?”

Clarified workflow:

1. Insert phagemid into bacteria
2. Add helper phage
3. Helper phage provides:
   • structural proteins
   • replication machinery

👉 Phage particles produced containing:

• your DNA
• your displayed protein

💡 “Why 2 types of genome?”

✔️ Needed because:

Phagemid alone: ❌ cannot form phage

Helper phage: ✔️ provides missing genes

💡 “2 types of pIII?”

✔️ Yes:

• Wild-type pIII → ensures infection works
• Fusion pIII → displays your protein

⚠️ 22. Limitations of Phage Display

💡 “Not suitable for PTM?”

✔️ Correct:

Bacteria cannot:

• glycosylate
• perform complex folding

👉 Result:

• protein may not behave naturally

🍞 23. Yeast Display — Deeper

💡 “Fusion protein on surface?”

✔️ Mechanism:

• Aga1 = anchored in wall
• Aga2 = binds Aga1
• Your protein fused to Aga2

👉 Result:

• protein displayed externally

💡 “Covalent interaction?”

✔️ Yes:

• Aga1–Aga2 linked via disulfide bond

💡 “Yeast diffused by proteins?”

Better interpretation:

👉 Yeast surface is densely coated with your protein

🧪 24. Why Yeast is Better for PTMs

✔️ Yeast:

• has ER + Golgi
• performs:
  • glycosylation
  • disulfide bonds

❌ Bacteria:

• lacks these systems

🔬 25. FACS — Detailed Mechanism

💡 “How sorting works?”

1. Cells flow single-file
2. Laser excites fluorophore
3. Detector measures signal
4. Droplet charged based on signal
5. Electric field sorts droplets

💡 “Does it use droplets?”

✔️ Yes:

• each cell → one droplet

💡 “Excite fluorophore?”

✔️ Laser → excites electrons → emits light → detected

🧬 26. Yeast Library Limitation

💡 “Why limited?”

• Sorting speed is bottleneck
• Must analyze one cell at a time

👉 Therefore:

• max ~10⁶–10⁷ variants

🦠 27. Bacterial Display — Clarified

💡 “Protein on membrane?”

✔️ Yes:

Proteins fused to:

• outer membrane proteins
• transporters
• surface structures

💡 Differences vs yeast:

🧠 Final Conceptual Summary

Core principle across all systems:

👉 You must:

1. Display protein
2. Keep DNA attached
3. Select based on function
4. Recover DNA

Systems comparison (conceptual):

🧬 1. Selection vs Screening (Core Concept You Didn’t Explicitly Ask)

🔍 Screening

• Test each variant individually
• Very slow and labor-intensive

⚡ Selection

• Only variants with desired function survive or get enriched
• Much faster

👉 Key idea: Selection filters automatically, screening tests manually

📊 2. Library Size & System Trade-offs

Different systems allow different library complexities:

💡 Important insight:

👉 Removing cells = massive increase in diversity

🧪 3. In Vivo vs In Vitro Selection Systems

🧫 In vivo (cell-based)

• Occurs inside living cells
• Examples:
  • bacteria
  • yeast
  • phage

✔️ Advantages:

• proper folding (sometimes)
• biological relevance

❌ Limitations:

• smaller libraries

🧪 In vitro (cell-free)

• occurs in test tube

✔️ Advantages:

• huge libraries
• no transformation limit

❌ Limitations:

• no cellular machinery

🧬 4. Droplet-Based Screening (Mentioned Briefly in File)

💡 Concept:

• Each cell is isolated in a microdroplet
• Acts like a tiny reaction chamber

Why useful:

• Allows high-throughput screening
• Maintains genotype–phenotype link

Compared to FACS:

• FACS → sorts cells
• Droplets → allow reactions per cell

🧬 5. Historical Breakthrough: Phage Display (George P. Smith)

Key contribution:

• First to physically link genotype and phenotype

👉 Inserted foreign DNA into phage coat protein

Result:

• Protein displayed outside
• DNA inside

Why revolutionary:

• Enabled evolution of proteins in the lab

🧱 6. Phage Structure & Protein Roles (Expanded View)

You saw the main ones, but here’s the full logic:

Structural proteins:

• p3 → infection
• p6 → structural end
• p7 + p9 → assembly initiation
• p8 → main coat

Functional proteins:

• replication proteins
• assembly proteins
• DNA processing proteins

👉 Total:

• ~10 genes → 11 proteins

🧬 7. Assembly of Phage (Important Mechanistic Detail)

Key concept:

Assembly is sequential and directional

Steps:

1. DNA recognized at origin
2. p7/p9 start assembly
3. p8 coats DNA
4. p3/p6 terminate assembly

👉 This ensures:

• correct orientation
• functional phage

🧪 8. Leader Sequence (Signal Peptide)

💡 What is it?

• Short sequence at N-terminus
• Directs protein to periplasm

Why needed?

👉 Fusion proteins must reach:

• membrane
• or periplasm

→ for incorporation into phage

🧬 9. Promoters & Expression Control

Phagemid contains:

• promoter
• ribosome binding site

Why important?

Controls:

• how much protein is produced
• timing of expression

👉 Too much expression: ❌ toxic ❌ misfolding

🧬 10. Transformation Efficiency

💡 Why vector size matters:

Large DNA:

• harder to enter cells
• reduces library diversity

Practical consequence:

👉 Smaller vectors = better libraries

🧬 11. Multivalent vs Monovalent Display

Multivalent:

• many copies per particle

✔️ stronger binding ❌ masks affinity differences

Monovalent:

• one copy per particle

✔️ accurate affinity measurement ❌ weaker signal

🧬 12. Avidity Effect (Expanded)

💡 What is it?

Multiple weak interactions combine → strong overall binding

Why problematic?

👉 Cannot distinguish:

• strong binder (true affinity)
• weak binder (many interactions)

🧬 13. Protein Size Constraints in Display

Observation:

• Small proteins easier to display
• Large proteins may:
  • misfold
  • block assembly

System differences:

🧬 14. Expression Localization (Periplasm vs Cytoplasm)

Why periplasm?

• oxidizing environment
• allows disulfide bonds

Importance:

Critical for:

• antibody fragments
• secreted proteins

🧬 15. Selection Workflow (Generalized)

Across all systems:

Step-by-step:

1. Create library
2. Express/display proteins
3. Apply selection pressure
4. Isolate binders
5. Amplify DNA
6. Repeat (iterative evolution)

👉 This is directed evolution

🧬 16. Fusion Protein Design Principles

Important constraints:

• must not disrupt:
  • folding
  • function
  • display system

Use of linkers:

• flexible (Gly-Ser)
• prevent steric clashes

🧬 17. Why Selection Can Be Repeated (Enrichment)

Each round:

• enriches best binders

Result:

• exponential enrichment of good variants

🧬 18. Surface Density Differences

👉 Implication:

• high density → strong signal
• but increases avidity

🧬 19. Carrier Proteins in Bacterial Display

Different strategies:

• outer membrane proteins
• lipoproteins
• autotransporters

Purpose:

Anchor foreign protein to surface

🧠 Final Big Picture (Unifying Concept)

All systems solve the same problem:

👉 “How do we connect function to DNA so we can evolve proteins?”

Strategy:

⚡ Most Important Insights to Remember

• Genotype–phenotype linkage is EVERYTHING
• Phage = high diversity, low biological realism
• Yeast = lower diversity, higher biological realism
• Avidity can mislead selection
• Vector size limits library quality
• PTMs determine system choice