Structural and functional regulation of Chlamydomonas lysosome-related organelles during environmental changes.

Lysosome-related organelles (LROs) are a class of heterogeneous organelles conserved in eukaryotes that primarily play a role in storage and secretion. An important function of LROs is to mediate metal homeostasis. Chlamydomonas reinhardtii is a model organism for studying metal ion metabolism; however, structural and functional analyses of lysosome-related organelles in C. reinhardtii are insufficient. Here, we optimized a method for purifying these organelles from two populations of cells: stationary phase or overloaded with iron. The morphology, elemental content, and lysosomal activities differed between the two preparations, even though both have phosphorus and metal ion storage functions. LROs in stationary-phase cells had multiple non-membrane-bound polyphosphate granules to store phosphorus. Those in iron-overloaded cells were similar to acidocalcisomes, which have a boundary membrane and contain one or two large polyphosphate granules to store more phosphorus. We established a method for quantifying the capacity of lysosome-related organelles to sequester individual trace metals. Based on a comparative proteomic analysis of these two types of LROs, we present a comprehensive acidocalcisome proteome and identified 113 putative acidocalcisome proteins. The methods and protein inventories provide a framework for studying the biogenesis and modification of lysosome-related organelles and the mechanisms by which they participate in regulating metal-ion metabolism.