25.10
Introdução à actina
A actina é uma proteína do citoesqueleto altamente conservada, encontrada abundantemente em células eucarióticas. Ela constitui 10% do peso da proteín…
No citoesqueleto, as actinas são os blocos de construção dos microfilamentos do citoesqueleto. Os monômeros de actina têm uma forma redonda e são chamados globulares ou G-actina.
Esses monômeros polimerizam da cabeça à cauda formando uma estrutura helicoidal apertada e destra chamada filamentosa ou F-actina.
Cada subunidade de actina tem um domínio externo e um interno unidos por uma hélice ligante menor. Tal arranjo forma duas fendas: a fenda superior se liga a um ATP e a um íon magnésio. A fenda hidrofóbica inferior é específica para proteínas de ligação à actina.
A G-actina tem uma baixa atividade de ATPase que é aumentada na F-actina. Quando um complexo ATP-G-Actina se liga à F-actina, o ATP é hidrolisado em ADP e fosfato, formando um filamento altamente estável.
A extremidade crescente ligada ao ATP da F-actina é chamada de extremidade positiva, enquanto a outra extremidade, ligada ao ADP, é a extremidade negativa.
As actina têm diferentes isoformas, amplamente classificadas em alfa, beta e gama. Eles são expressos em diferentes tipos de células, como α-actina nas fibras musculares contráteis, β-actina no córtex celular e γ-actina nas fibras musculares lisas.
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Q1: What is the difference between G-actin and F-actin?
G-actin (globular actin) refers to individual actin monomers with a round shape, while F-actin (filamentous actin) forms when G-actin monomers polymerize head-to-tail into a tight, right-handed helical structure. F-actin has enhanced ATPase activity compared to the low activity of G-actin, and this structural transformation is fundamental to actin polymerization.
Q2: How does ATP binding affect actin filament stability?
When ATP-bound G-actin binds to F-actin, the ATP is hydrolyzed to ADP and phosphate, forming a highly stable filament. The ATP-bound growing end is called the plus-end, while the ADP-bound end is the minus-end. This ATP hydrolysis is critical for maintaining filament stability and polarity.
Q3: What structural features allow actin to bind other proteins?
Each actin subunit contains an outer and inner domain connected by a linker helix, creating two clefts. The upper cleft binds ATP and magnesium ions, while the lower hydrophobic cleft is specific for actin-binding proteins. This arrangement enables actin to interact with diverse regulatory and structural proteins.
Q4: What are the three main classes of actin isoforms and where are they expressed?
Actin isoforms are classified into alpha, beta, and gamma types based on their isoelectric points. Alpha-actin is expressed in contractile muscle fibers, beta-actin in the cell cortex, and gamma-actin in smooth muscle fibers. These tissue-specific isoforms are nearly identical but have distinct cellular roles.
Q5: Why is actin considered a highly conserved protein across species?
Actin found in unicellular amoebae and complex multicellular animals is approximately 80% similar, demonstrating conservation over a billion years of evolution. Additionally, actins in yeast and humans share 87% similarity. This conservation reflects actin's fundamental importance in cellular functions across diverse organisms.
Q6: What percentage of total cellular protein does actin represent in different cell types?
In muscle cells, actin constitutes approximately 10% of total cellular protein by weight, making it exceptionally abundant. In non-muscle cells, actin comprises only 1-5% of total cellular protein. This difference reflects the distinct metabolic demands and structural requirements of contractile versus non-contractile cells.
Q7: How do actin filaments contribute to cellular processes beyond muscle contraction?
Beyond muscle contraction, actin filaments play essential roles in cell migration, cell adhesion, cell division, protein trafficking, and membrane organization. The discovery of actin in non-muscle cells during the early 1970s revealed its broad importance in the role of actin and myosin in non-muscle cells.