Phospholamban

Phospholamban (PLN) is a small protein that plays a crucial role in cardiac muscle contraction and relaxation. It is primarily found in the sarcoplasmic reticulum (SR), a specialized type of endoplasmic reticulum in muscle cells.

Structure

Phospholamban is a pentameric protein, meaning it is composed of five identical subunits. Each subunit consists of 52 amino acids and has a molecular weight of approximately 6 kDa. The protein has both hydrophilic and hydrophobic regions, allowing it to interact with both the aqueous environment of the cell and the lipid bilayer of the SR membrane.

Function

Phospholamban regulates the activity of the sarcoplasmic reticulum calcium ATPase (SERCA), an enzyme that pumps calcium ions (Ca2+) from the cytosol of the cell into the SR. By inhibiting SERCA, phospholamban decreases the rate of Ca2+ uptake into the SR, thereby slowing the relaxation of the cardiac muscle after contraction.

When phospholamban is phosphorylated by protein kinase A (PKA) or calcium/calmodulin-dependent protein kinase II (CaMKII), it releases its inhibition on SERCA, allowing for faster Ca2+ uptake and quicker muscle relaxation. This phosphorylation is a key part of the fight-or-flight response, as it allows the heart to beat faster and stronger in response to stress.

Clinical significance

Mutations in the gene encoding phospholamban can lead to various forms of cardiomyopathy, a disease of the heart muscle. For example, a mutation that prevents phospholamban from being properly phosphorylated can lead to dilated cardiomyopathy, a condition in which the heart becomes enlarged and cannot pump blood efficiently.

See also

• Cardiac muscle
• Sarcoplasmic reticulum
• Sarcoplasmic reticulum calcium ATPase
• Protein kinase A
• Calcium/calmodulin-dependent protein kinase II
• Cardiomyopathy