Newly synthesized proteins in the lumen and the membrane of the endoplasmic reticulum are transported to the Golgi apparatus, where posttranslational modification of proteins and lipids occurs, commonly by the addition of carbohydrate residues. The Golgi apparatus is also the major sorting center of the cell; proteins proceed from this organelle to lysosomes, secretory granules, or the plasma membrane, according to signals encoded within their amino acid sequences and three-dimensional structures. A growing number of resident membrane proteins have been identified in the last few years, but the function of many of these proteins remains poorly characterized to date. In this issue, as a continuation of their previous studies, Fimmel and colleagues report that GP73 protein is overexpressed in a variety of acute and chronic liver diseases. In contrast to their previous studies, they show expression of GP73 in sinusoidal lining cells, and suggest that activated stellate cells may be a potential source of this protein. In the current study, the authors do not provide any new insight on the function and role of this protein in liver disease. The widespread expression of this protein in many diverse acute and chronic conditions suggests that the measurement of this protein is unlikely to be very useful for etiological diagnosis or staging, but it may prove to be a marker of liver disease. However, a better understanding of the role of this protein may provide more insights into the pathogenesis of liver injury and progression.
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