Cellular Layer — How Cells Negotiate and Confirm Signals
Cells constantly exchange information, but communication is not automatic.
Instead, multi-step cellular protocols govern who gets heard, when, and how — involving receptors, co-receptors, and environment-dependent modifications.
Example 1: Plant PRR-BAK1 Complex — A Structured Handshake for Immunity
In plant innate immunity, Pattern Recognition Receptors (PRRs) detect PAMPs (Pathogen-Associated Molecular Patterns) like bacterial flagellin.
But recognition alone is not enough — co-receptor BAK1 must join for the signal to proceed, forming a multi-step handshake.
Protocol Breakdown (step-by-step):
| Step | Action |
|---|---|
| Recognition | FLS2 binds flg22 (PAMP) — packet reception. |
| Negotiation | BAK1 joins only if FLS2-flg22 complex is formed — handshake validation. |
| Execution | Kinase cascade (BIK1, MAPKs) propagates the immune signal. |
| Confirmation | Scaffold proteins stabilize the receptor complex. |
| Error handling | BIR proteins prevent false activations. |
Failure Example:
- Pathogen effectors (e.g., AvrPto) block BAK1 recruitment — handshake hijacked, immune response blocked.
Example 2: Mammalian Immune Synapse — Stepwise Confirmation to Prevent Error
T cell activation requires multiple confirmations — a complex negotiated immune protocol.
- Recognition: TCR binds antigen on MHC.
- Negotiation: Co-receptors (CD4 or CD8) confirm MHC type.
- Co-stimulation: CD28 binds B7 — essential handshake.
- Execution: Intracellular signaling (ZAP70, LAT).
- Error handling: CTLA-4 can terminate the process if activation is inappropriate.
Failure Example:
- If checkpoints (like CTLA-4) fail, autoimmune diseases like lupus or rheumatoid arthritis can arise — protocol breakdown.
Key Takeaways for Cellular Layer
- Cell-cell communication is a negotiated, multi-step protocol, not a simple binding event.
- Co-receptors and scaffolds act as protocol managers, confirming and stabilizing interactions.
- Error handling mechanisms prevent false signaling — when these fail, disease follows.
- Thinking in protocols explains why immune responses are specific, and how pathogens exploit these protocols to avoid detection.
