Actin Educational Materials

University:
The Pennsylvania State University
Course:
BMB 252 | Molecular and Cell Biology II
Academic year:

2024
Views:

81

Pages:

3
Author:

Alexis H.

Actin Side notes: Actin filaments Actin subunit is an actin monomer Each actin subunit has a binding site for ATP Actin subunits assemble head to tail Actin filaments are polar (plus end is different from minus) Actin filaments assemble from two parallel protofilaments that are twisted around each other in a right-handed helix Actin filaments are relatively flexible and easier to bend than microtubules ATP/GTP hydrolysis Free floating, cytoplasmic actin subunits have ATP; tubular subunits have GTP Upon addition to filament, ATP/GTP is hydrolyzed to an ADP/GDP The hydrolysis is associated with a conformational change Subunits with ADP/GDP are easier to dissociate What happens to a filament during treadmilling? 1. 2. 3. It dissociates It grows It moves Treadmilling At some concentration of free subunits, the plus end grows and the minus end shrinks at the same rate The length of the filament stays the same This is called treadmilling Treadmilling can occur for both actin filaments and microtubules Actin filaments nucleation (starting a new filament from subunits) Occurs at or near the plasma membrane The highest density of actin is at the cell periphery The layer beneath the cell membrane is called the cell cortex The actin filaments in the cell cortex determine the shape and movement of the cell surface ARP and formins promote actin nucleation External signals regulate actin nucleation Nucleation of actin is catalyzed by two different types of regulated factors: the ARP complex (actin related proteins) and the forming ARPs mostly catalyze the formation of branched structures, while formins catalyze the straight, non branched actin filaments, which can form parallel bundles ARPs ARPs are 45% identical to actin ARP complex nucleates actin at the minus end, allowing the plus end to grow ARP complex can also attach to side of the actin filament building a tree like web ARPs is associated with the structures at the leading edge of migrating cells Formins can stabilize actin filaments by forming a ring over the plus end Formin travels with the elongating plus end Many formins are indirectly connected to the cell plasma membrane Actin Side notes: Proteins that interact with filament ends In some cases, the actin filaments should be stabilized from degradation They are stabilized by binding capping proteins at the end, which does not allow it to shrink or grow ARP complex and gamma - TURC can serve as capping proteins for minus end In muscle cells the actin filament are long-lived and bound to capping proteins from both plus and minus ends