Identification and characterization of calreticulin as a novel plasminogen receptor.

Calreticulin (CRT) was originally identified as a key calcium-binding protein of the endoplasmic reticulum. Subsequently, CRT was shown to possess multiple intracellular functions, including roles in calcium homeostasis and protein folding. Recently, several extracellular functions have been identified for CRT, including roles in cancer cell invasion and phagocytosis of apoptotic and cancer cells by macrophages. In the current report, we uncover a novel function for extracellular CRT and report that CRT functions as a plasminogen-binding receptor that regulates the conversion of plasminogen to plasmin. We show that human recombinant or bovine tissue-derived CRT dramatically stimulated the conversion of plasminogen to plasmin by tissue plasminogen activator or urokinase-type plasminogen activator. Surface plasmon resonance analysis revealed that CRT-bound plasminogen (KD = 1.8 µM) with moderate affinity. Plasminogen binding and activation by CRT were inhibited by ε-aminocaproic acid, suggesting that an internal lysine residue of CRT interacts with plasminogen. We subsequently show that clinically relevant CRT variants (lacking four or eight lysines in carboxyl-terminal region) exhibited decreased plasminogen activation. Furthermore, CRT-deficient fibroblasts generated 90% less plasmin and CRT-depleted MDA MB 231 cells also demonstrated a significant reduction in plasmin generation. Moreover, treatment of fibroblasts with mitoxantrone dramatically stimulated plasmin generation by WT but not CRT-deficient fibroblasts. Our results suggest that CRT is an important cellular plasminogen regulatory protein. Given that CRT can empower cells with plasmin proteolytic activity, this discovery may provide new mechanistic insight into the established role of CRT in cancer.

Calreticulin-Enigmatic Discovery.

Calreticulin (CRT) is an intrinsically disordered multifunctional protein that plays essential roles intra-and extra-cellularly. The Michalak laboratory has proposed that CRT was initially identified in 1974 by the MacLennan laboratory as the high-affinity Ca2+-binding protein (HACBP) of the sarcoplasmic reticulin (SR). This widely accepted belief has been ingrained in the scientific literature but has never been rigorously tested. In our report, we have undertaken a comprehensive reexamination of this assumption by meticulously examining the majority of published studies that present a proteomic analysis of the SR. These analyses have utilized proteomic analysis of purified SR preparations or purified components of the SR, namely the longitudinal tubules and junctional terminal cisternae. These studies have consistently failed to detect the HACBP or CRT in skeletal muscle SR. We propose that the existence of the HACBP has failed the test of reproducibility and should be retired to the annals of antiquity. Therefore, the scientific dogma that the HACBP and CRT are identical proteins is a non sequitur.

Recent Advances in Molecular and Cellular Functions of S100A10.

S100A10 (p11, annexin II light chain, calpactin light chain) is a multifunctional protein with a wide range of physiological activity. S100A10 is unique among the S100 family members of proteins since it does not bind to Ca2+, despite its sequence and structural similarity. This review focuses on studies highlighting the structure, regulation, and binding partners of S100A10. The binding partners of S100A10 were collated and summarized.