However, this regulated mechanism for vacuolar degradation was limited only to a small and specific group of proteins (see for Belinostat buy example Müller et al.18; reviewed in Holzer19). More recent studies have shown that at least for stress-induced macroautophagy, a general sequence of amino acids, KFFERQ, directs, via binding to a specific “receptor” and along with cytosolic and lysosomal chaperones, the regulated entry of many cytosolic proteins into the lysosomal lumen. While further corroboration of this hypothesis

is still required, it can only explain the mass entry of a large population of proteins that contain a homologous sequence, but not Inhibitors,research,lifescience,medical the targeting for degradation of a specific protein under defined conditions (reviewed Inhibitors,research,lifescience,medical in Majeski et al.20 and Cuervo et al.21). The energy requirement for protein degradation was described as indirect, and necessary, for example, for protein transport across the lysosomal membrane22 and/or for the activity of the H+ pump and the maintenance of the low acidic intralysosomal pH that is necessary for optimal activity of the proteases.23 We now know that both mechanisms require energy. In the absence of any alternative, and with lysosomal degradation as the most logical explanation for targeting all known classes of proteins at the time, Christian

de Duve Inhibitors,research,lifescience,medical summarized his view on the subject in a review article published in the mid-1960s, saying: “Just as extracellular besides digestion is successfully carried out by the concerted action of enzymes with limited individual capacities, so, we believe, is intracellular digestion.”24 The problem of different sensitivities of distinct protein groups to lysosomal inhibitors has remained unsolved Inhibitors,research,lifescience,medical and may have Inhibitors,research,lifescience,medical served as an important trigger in the future quest for a non-lysosomal proteolytic system. Progress in identifying

the elusive, non-lysosomal proteolytic system(s) was hampered by the lack of a cell-free preparation that could faithfully replicate the cellular proteolytic events—i.e. degrading proteins in a specific and energy-requiring mode. An important breakthrough Dacomitinib was made by Rabinovitz and Fisher who found that rabbit reticulocytes degrade abnormal, amino acid analog-containing hemoglobin.25 Their experiments modeled known disease states, the hemoglobinopathies. In these diseases abnormal mutated hemoglobin chains (such as sickle cell hemoglobin) or excess of unassembled normal hemoglobin chains (which are synthesized normally, but also excessively in thalassemias, diseases in which the pairing chain is not synthesized at all or is mutated and rapidly degraded, and consequently the bi-heterodimeric hemoglobin complex is not assembled) are rapidly degraded in the reticulocyte.26,27 Reticulocytes are terminally differentiating red blood cells that do not contain lysosomes.