Structure and Interactions of the Extracellular Matrix Protein PRELP

Abstract: This thesis describes the structure and interactions of a novel extracellular matrix protein - PRELP (proline arginine-rich end leucine-rich repeat protein). The amino acid sequence of PRELP showed that it belonged to the leucine-rich repeat (LRR) protein family in connective tissues. The main part of PRELP is composed of 10 LRRs flanked by disulfide bridges at the amino- and carboxy-terminal ends, respectively. Most of the other members in the family have a similar structure, but their amino-terminal domains outside the disulfide bridged loop differ markedly. The amino-terminal region of PRELP is unique in being rich in prolines and has clusters of basic residues. This region was shown to bind heparin and heparan sulfate (HS). The PRELP-heparin interaction was dependent on the number of sulfate groups present on heparin. In tissues, HS is mostly found in proteoglycans in basement membranes (BM) and on the cell surface. The amino-terminal region of PRELP bound the HS chains of the BM proteoglycan perlecan. Immunohistological studies of skin, testes and kidney showed that PRELP was localized in the connective tissue adjacent to the BM. The LRR-containing domain of PRELP interacted with fibrillar collagens present in connective tissues. This suggests that PRELP may function in anchoring the BMs to the underlying connective tissue. In a cell adhesion assay rat skin fibroblasts bound to PRELP. The interaction was mediated by proteoglycans as the interaction was inhibited with i) soluble heparin and HS, ii) chlorate treatment of fibroblasts, which inhibits the sulfation of proteins and glycosaminoglycans, and iii) a combination of heparinase and chondroitinase digestion of the fibroblasts. Using affinity chromatography, a 35S- labeled 150-230 kDa component was isolated from rat skin fibroblasts binding to a PRELP column.