Actin and Myosin

ID #2467

So under "in vivo circumstances" polymerization always occurs at the + end and depolymerization always occurs at the - end. And with "in vitro circumstances" it can vary?

No, it's not quite that simple. Since "in vivo" means "in life" or in living cells, the cellular environment (and the available actin) can change, especially in response to the environment. 
The actual concentration of G actin available will vary based on many factors - so there can be times when: 
- both ends are polymerizing 
- one end is polymerizing but the other end is adding new G-actin as quickly as G-actin is leaving, resulting in no net growth 
- both ends are depolymerizing 
The best way to think about problems involving actin and polymerizing/depolymerizing is to first consider the critical concentrations at both the plus and minus end. Then, consider what the concentration of available actin in the environment (whether it's in vitro or in vivo) and go from there. On an exam question, this information would be provided to you.

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