Question

What are the different import/export and secretion systems in plasma membrane and outer membrane. Then describe them.

Answer 1

Export- Endocytosis,

Import- exocytosis

Secretion- the different secretion system include  8 types of SS (T1SS, T2SS, T3SS, T4SS, T5SS, T6SS, T7SS, and T9SS)

Endocytosis-

• Nutrients - Cells transport nutrients across cell membranes into specific compartments for cellular use.For example - Insulin receptor, glucagon receptor and glucose transporter embedded in the plasma membrane.

Exocytosis-

• Cell Secretion and Waste - Cells transport secretory products, signalling molecules and excretory products across cell membranes into extracellular specific compartments. For example - extracellular matrix, hormones, neurotransmitters are secreted by cells. Three pathways of exocytosis are constitutive exocytosis, regulated exocytosis, and lysosome mediated exocytosis. Steps of exocytosis include vesicle trafficking, tethering, docking, priming, and fusing.

Secretion-

The type I pathway is exemplified by hemolysin A (HlyA) secretion in E. coli where TolC, HlyD, and HlyB are the three components which constitute the channel to transport HlyA to extracellular space. Sec (general secretion route) and Tat (twin-arginine translocation pathway) transfer the substrates of T2SS and T5SS across the inner membrane. Sec also participates in the transport of T9SS substrates across de inner membrane. T9SS is also called Por Secretion System (PoSS). T4SS is represented by VirB/D system of Agrobacterium tumefaciens. The T7SS is based on a system in Mycobacteria. Red squares represent the ATPases. HM, host membrane; EM, extracellular medium; OM, outer membrane; IM, inner membrane; MM, mycomembrane.