The role of SPARC in extracellular matrix assembly

SPARC is a collagen-binding matricellular protein. Expression of SPARC in adult tissues is frequently associated with excessive deposition of collagen and SPARC-null mice fail to generate a robust fibrotic response to a variety of stimuli. This review summarizes recent advancements in the characterization of the binding of SPARC to collagens and describes the results of studies that implicate a function for SPARC in the regulation of the assembly of basal lamina and fibrillar collagen in the ECM. Potential cellular mechanisms that underlie SPARC activity in ECM deposition are also explored

Procollagen Triple Helix Assembly: An Unconventional Chaperone-Assisted Folding Paradigm

Fibers composed of type I collagen triple helices form the organic scaffold of bone and many other tissues, yet the energetically preferred conformation of type I collagen at body temperature is a random coil. In fibers, the triple helix is stabilized by neighbors, but how does it fold? The observations reported here reveal surprising features that may represent a new paradigm for folding of marginally stable proteins. We find that human procollagen triple helix spontaneously folds into its native conformation at 30–34°C but not at higher temperatures, even in an environment emulating Endoplasmic Reticulum (ER). ER-like molecular crowding by nonspecific proteins does not affect triple helix folding or aggregation of unfolded chains. Common ER chaperones may prevent aggregation and misfolding of procollagen C-propeptide in their traditional role of binding unfolded polypeptide chains. However, such binding only further destabilizes the triple helix. We argue that folding of the triple helix requires stabilization by preferential binding of chaperones to its folded, native conformation. Based on the triple helix folding temperature measured here and published binding constants, we deduce that HSP47 is likely to do just that. It takes over 20 HSP47 molecules to stabilize a single triple helix at body temperature. The required 50–200 µM concentration of free HSP47 is not unusual for heat-shock chaperones in ER, but it is 100 times higher than used in reported in vitro experiments, which did not reveal such stabilization

Regulation of the extracellular matrix by heat shock proteins and molecular chaperones:

The extracellular matrix (ECM) serves as a scaffold for cells within tissues and is composed of an intricate network of glycoproteins, growth factors and matricellular proteins which cooperatively function in cell processes such as migration, adhesion and wound healing. ECM morphology is constantly undergoing remodelling (synthesis, assembly and degradation) during normal cell processes and when deregulated may contribute to disease. Heat shock proteins (Hsps) are involved in regulating processes that determine the assembly and degradation of the ECM at multiple levels, in both normal and diseased states. These roles include mediating the activation of ECM-degrading enzymes, maintaining matrix stability and clearing aggregated/misfolded proteins. Hsp may serve as chaperones and receptors or have cytokine-like functions. In this chapter, we review how Hsp90, Hsp70, Hsp40 and a number of ER resident chaperones contribute to ECM regulation. The role of the non-Hsp chaperones, SPARC and clusterin in the ECM is also discussed