Endosome maturation: insights from somatic cell genetics and cell-free analysis.

Endosomal density changes in cell-free systems The mechanism by which dense lysosomes are generated in animal cells has been the subject of intense research and speculation (for reviews, see [ 1-31). Despite much progress in understanding the biosynthesis and trafficking of lysosomal enzymes, the internalization of ligands and solutes and the sorting of receptors and other molecules that are recycled to the plasma membrane from ligands and of other material that is destined for degradation in lysosomes, little is known about the process by which lysosomes acquire one of their distinguishing characteristics: their high density. This characteristic is reflected in an elevated electron, optical and buoyant density, compared with other endocytic compartments and with most membranecontaining organelles (for a historical review, see [4]). The exploration of the mechanism by which this phenomenon occurs has been complicated by the long-standing uncertainty over the source of the membrane and soluble content of lysosomes. Initially, endosomes were proposed either to be stable compartments that communicate with lysosomes by means of vesicle shuttles, or to be transient compartments that fuse with lysosomes [5]. After the discovery that lysosomal hydrolases are found in significant amounts in endosomes [6-91, it was suggested that transient endosomes could be converted directly into lysosomes [9, 101. The arguments for stable and for transient endosomes have been extensively discussed [ l l , 121, and further experimentation is clearly required to resolve this question. It is entirely possible that both mechanisms may occur and that the balance between them may be regulated by the growth state or by differentiation. In any case, the mechanism by which lysosomes become or remain denser than other endocytic structures remains unknown. Two classes of proposals for this mechanism have been put forth. The first, which was implicit in the notion of primary lysosomes, is that lysosomes are intrinsically dense (that is, created with a higher