This question tests whether you can tell apart proteins that live outside the cell, in the extracellular matrix (ECM), from proteins with similar sounding names that actually work inside the cell.
- Myosin: this is a cytoskeletal motor protein. It walks along actin filaments inside the cytoplasm to generate force, so it never leaves the cell to join the ECM. Not an ECM protein.
- Fibronectin: cells secrete this glycoprotein into the space around them. It has separate binding sites for collagen, fibrin and the integrin receptors on the cell surface, so it physically bridges the cell to its matrix. This is a true ECM protein.
- Lamin: easy to confuse with laminin, but it is a different molecule. Lamin is an intermediate filament protein that builds the nuclear lamina on the inner face of the nuclear envelope, so it stays inside the nucleus and is not part of the ECM.
- Collagen: the most abundant fibrous protein secreted into the ECM. Its triple helix structure packs into strong fibers that give skin, bone and cartilage their tensile strength. This is a true ECM protein.
Only fibronectin and collagen are genuinely secreted structural or adhesive components of the extracellular matrix. Myosin works inside the cytoplasm and lamin works inside the nucleus, so neither one qualifies.
Let's summarize:
- ECM proteins are secreted out of the cell: collagen (structural fiber), fibronectin and laminin (adhesive glycoproteins), elastin, proteoglycans.
- Myosin and lamin sound similar to ECM names but function entirely inside the cell.
So the correct answers are Fibronectin and Collagen, options (B) and (D).