Structural and functional characterization of the extracellular matrix proteins COMPcc and NtA

Abstract

The extracellular matrix (ECM) is a complex of proteins and carbohydrates that not only provides a structural support between tissues but also functions in a wide variety of cellular activities. ECM proteins are large, complex proteins with many repeating domains. Individual domains can be analyzed for the investigation of structure-function relationships. This thesis focuses on two ECM proteins of interest: N-terminal agrin (NtA) and the coiled-coil domain of cartilage oligomeric matrix protein (COMPcc). Agrin is an important ECM protein involved in postsynaptic differentiation at the neuromuscular junction, mediated by binding in the NtA domain. In agrin, the NtA domain is followed by nine follistatin-like (FS) domains. Structural studies showed a novel interdomain disulfide bridge between the NtA and first FS domain in agrin. This disulfide bridge compensates for a seven residue splice insert in the C-terminus of NtA, suggesting that the interdomain disulfide bond may be necessary for the proper folding of agrin. COMP is another important ECM protein that is found in cartilage, tendon, and ligament. It is a homopentamer held together by disulfide bonds in the central coiled-coil oligomerization domain. Previous structural studies demonstrated that COMPcc forms a pentameric α-helical coiled-coil structure containing a 73Å-long cavity with a diameter of 2–6Å that is capable of binding hydrophobic compounds. This binding capacity of COMPcc was investigated and the high-resolution crystal structures of COMPcc in complex with five naturally-occurring fatty acids were solved. Additionally, the binding properties of COMPcc in solution were investigated through the use of fluorescence spectroscopy. Both the x-ray crystallographic and solution data reveal that binding favourability of fatty acids to COMPcc is driven by length of the methylene tail and degree of unsaturation. These results suggest the possibility of COMPcc to be used in targeted drug delivery systems.