Selective proteolysis: 70-kDa heat-shock protein and ubiquitin-dependent mechanisms?

Abstract

The proteolysis of the vast majority of cytoplasmic proteins, once inside the lysosome compartment, would be expected to occur rapidly; therefore, the rate-limiting step in the lysosomal degradation of cytoplasmic proteins must be their initial sequestration. Indeed, the initial sequestration serves to remove cytoplasmic proteins from their functional site in the cell. Bulk sequestration of cytoplasm (macro-autophagy), described elsewhere in this volume (Chapter 4), is considered essentially nonselective. Microautophagy has been described by a number of investigators over the years (see Chapter 4) and may provide a mechanism for the selective sequestration of cytoplasmic proteins into the lysosome system. However, controversy surrounds the whole topic of selective lysosomal proteolysis, with some investigators probably of the view that the lysosome system is entirely nonselective. However, recent work from the laboratory of Dice (Dice, 1990) describes a selective mechanism of sequestration of cytosolic proteins into lysosomes for degradation in a process regulated by serum and involving a member of the 70-kDa heat-shock protein family.