Table 1.
Overview of protein secretion systems and the substrate features in Gram-negative bacteria.
| Secretion system | Secretion step(s) | Membrane spanning | Secretion signal | Substrate state |
|---|---|---|---|---|
| Sec | 1 | Inner | N-terminus | Unfolded |
| Tat | 1 | Inner | N-terminus | Folded |
| T1SS | 1 | Inner + Outer | C-terminus | Unfolded |
| T2SS | 2 (Sec/Tat) | Inner + Outer | N-terminus | Folded |
| T3SS1 | 1 or 2 (Sec) | Inner + Outer (+Host) | N-terminus | Unfolded |
| T4SS2 | 1 | Inner + Outer (+Host) | C-terminus | Unfolded |
| T6SS | 1 | Inner + Outer + Host | N-terminus? | Folded |
| T5SS | 2 (Sec) | Outer | N-terminus | Unfolded |
| Pili/ T7SS | 2 (Sec) | Outer | N-terminus | Folded |
| Curli/ T8SS | 2 (Sec) | Outer | N-terminus | Unfolded |
| T9SS | 2 (Sec) | Inner + Outer | C-terminus | Folded |
Notes: 1 T3SSs include non-flagella T3SSs and flagella T3SSs. Non-flagella T3SSs are translocation systems delivering substrates into host cells in one step, while flagella T3SSs involve two steps to secrete substrates extracellularly. 2 T4SSs translocate substrate proteins into host cells like T3SSs, or transport the proteins into extracellular milieu.