Acidic cytosolic proteins are preferentially imported into rat liver lysosomes

Previous studies have reported that lysosomes isolated from human diploid fibroblasts and from rat liver can selectively import and degrade specific proteins. We have now reinvestigated this selectivity using an in vitro assay with rat liver lysosomes and an extract of cytosolic proteins prepared from cultured cells labeled to equilibriums with [35S-]methionine. Analysis by two-dimensional gel electrophoresis and autoradiography of the cytosolic proteins bound to the lysosomal membrane and imported into the lysosomes shows that when all cytosolic proteins are simultaneously present in the in vitro assay the lysosomal uptake also occurs in a specific manner. These findings suggest that isolated lysosomes are able to discriminate among different proteins, selecting those with certain features for lysosomal degradation. Additional characterization of the cytosolic proteins which are selectively imported by lysosomes shows that a common structural feature of most, but not all, of these proteins is an acidic isoelectric point (pI <6.0) and a small or intermediate size. This observation is in agreement with earlier studies which established a relationship between the in vivo half-lives of cytosolic proteins in rat liver and their net charge, with acidic proteins, in general, being degraded more rapidly than neutral or basic proteins. The reasons for this preference are still uncertain, although a possible explanation is presented.